Abstract
De Corpore sets out Hobbes’s materialist first philosophy and physics: nothing exists but bodies in motion, and every phenomenon is to be explained mechanically through efficient causes. On names and universals Hobbes is a nominalist - general terms do not designate real separated essences but are names imposed on many things - and he attacks the metaphysics of substantial forms in Aristotle and the Schools. The method is modelled on geometry, proceeding by definitions, reasoning and demonstration from the first notions of body, motion and cause.
Testo originale inglese · Delphi Classics, 2019 · pubblico dominio
DEFINITIONS.
1 A SCALE, is a straight line, whose middle point is immoveable, all the rest of its points being at liberty; and that part of the Scale which reaches from the center to either of the waights, is called the Beam.
2 Equiponderation is, when the endeavour of one Body which presses one of the Beams, resists the endeavour of another Body pressing the other Beam, so, that neither of them is moved; and the Bodies when neither of them is moved, are said to be Equally poised.
3 Waight, is the aggregate of all the Endeavours, by which all the points of that Body which presses the Beam, tend downwards in lines parallel to one another; and the Body which presses, is called the Ponderant.
4 Moment, is the Power which the Ponderant has to move the Beam, by reason of a determined situation.
5 The plain of Equiponderation, is that, by which the Ponderant is so divided, that the Moments on both sides remain equal.
6 The Diameter of Equiponderation, is the common Section of the two Plains of Equiponderation; and is in the straight line by which the waight is hanged.
7 The Center of Equiponderation, is the common point of the two Diameters of Equiponderation.
SUPPOSITIONS
1 WHEN TWO Bodies are equally pois’d, if waight be added to one of them, and not to the other, their Equiponderation ceases.
2 When two Ponderants of equal magnitude, and of the same Species or matter, press the Beam on both sides at equal distances from the center of the Scale, their Moments are equal. Also when two Bodies endeavour at equal distances from the center of the Scale, if they be of equal magnitude and of the same Species, their Moments are equal.
2 No two Plains of Equiponderation are parallel.
Let A B C D (in the first figure) be any Ponderant whatsoever; and in it let E F be a Plain of Equiponderation; parallel to which, let any other Plain be drawn, as G H. I say G H is not a Plain of Equiponderation. For seeing the parts A E F D and E B C F of the Ponderant A B C D, are equally pois’d; and the weight E G H F is added to the part A E F D, and nothing is added to the part E B C F, but the weight E G H F is taken from it; therefore (by the first Supposition) the parts A G H D and G B C H will not be equally pois’d; and consequently G H is not a Plain of Equiponderation. Wherefore, No two Plains of Equiponderation, &c. Which was to be proved.
3 The Center of Equiponderation is in every Plain of Equiponderation.
For if another Plain of Equiponderation be taken, it will not (by the last Article) be parallel to the former Plain; and therefore both those Plains will cut one another. Now that Section (by the 6th Definition) is the Diameter of Equiponderation. Again, if another Diameter of Equiponderation be taken, it will cut that former Diameter; and in that Section (by the 7th Definition) is the Center of Equiponderation. Wherefore the Center of Equiponderation is in that Diameter which lies in the said Plain of Equiponderation.
4 The Moment of any Ponderant applyed to one point of the Beam, to the Moment of the same, or an equal Ponderant applyed to any other point of the Beam, is as the distance of the former point from the Center of the Scale, to the distance of the later point from the same Center. Or thus, Those Moments are to one another, as the Arches of Circles which are made upon the Center of the Scale through those points, in the same time. Or lastly thus; They are, as the parallel bases of two Triangles, which have a common angle at the Center of the Scale.
Let A (in the 2d figure) be the Center of the Scale; and let the equal Poderants D and E press the Beam A B in the points B and C; also let the straight lines B D and C E be Diameters of Equiponderation; and the points D and E in the Ponderants D and E be their Centers of Equiponderation. Let A G F be drawn howsoever, cutting D B produced in F, and E C in G; and lastly, upon the common Center A, let the two arches B H and C I be described, cutting A G F in H and I. I say the Moment of the Ponderant D to the Moment of the Ponderant E, is as A B to A C, or as B H to C I, or as B F to C G. For the effect of the Ponderant D in the point B, is circular motion in the arch B H; and the effect of the Ponderant E in the point C, circular motion in the arch C I; and by reason of the equality of the Ponderants D and E, these motions are to one another as the Quicknesses or Velocities with which the points B and C describe the arches B H and C I, that is, as the arches themselves B H and C I, or as the straight parallels B F and C G, or as the parts of the Beam A B and A C; for A B. A C:: B F. C G:: B H. C I. are proportionals; and therefore the effects, that is, (by the 4th Definition) the Moments of the equal Ponderants applyed to several points of the Beam, are to one another, as A B and A C; or as the distances of those points from the center of the Scale; or as the parallel bases of the Triangles which have a common angle at A; or as the concentrick arches B H and C I; which was to be demonstrated.
5 Unequal Ponderants, when they are applyed to several points of the Beam, and hang at liberty (that is, so as the line by which they hang be the Diameter of Equiponderation, whatsoever be the figure of the Ponderant), have their Moments to one another in proportion compounded of the proportions of their distances from the center of the Scale, and of their Waights.
Let A (in the 3d figure) be the center of the Scale, and A B the Beam; to which let the two Ponderants C & D be applied at the points B and E. I say the proportion of the Moment of the Ponderant C, to the Moment of the Ponderant D, is compounded of the proportions of A B to A E and of the Waight C to the Waight D; or (if C and D be of the same species) of the magnitude C to the magnitude D.
Let either of them, as C, be supposed to be bigger then the other D. If therefore by the addition of F, F and D together be as one Body equal to C, the Moment of C to the Moment of F + D will be (by the last article) as B G is to E H. Now as F + D is to D, so let E H be to another E I; and the moment of F + D, that is of C, to the moment of D, will be as B G to E I. But the proportion of B G to E I is compounded of the proportions (of B G to E H that is) of A B to A E, and (of E H to E I, that is) of the waight C to the waight D. Wherefore unequal Ponderants, when they are applied, &c. which was to be proved.
6 The same figure remaining, if I K be drawn parallel to the Beame A B, and cutting A G in K; and K L be drawn parallel to to B G, cutting A B in L, the distances A B and A L from the center, will be proportional to the moments of C and D. For the moment of C is B G, and the moment of D is E I, to which K L is equal. But as the distance A B from the center, is to the distance A L from the center, so is B G the moment of the Ponderant C, to L K, or E I the moment of the Ponderant D.
7 If two Ponderants have their waights and distances from the center in reciprocal proportion, and the center of the Scale be between the points to which the Ponderants are applied, they will be equally poised. And contrarily, if they be equally poised, their waights and distances from the center of the Scale will be in reciprocall proportion.
Let the center of the Scale (in the same 3d figure) be A, the Beam A B; and let any Ponderant C, having B G for its moment, be applied to the point B; also let any other Ponderant D, whose moment is E I, be applied to the point E. Through the point I, let I K be drawn parallel to the Beam A B, cutting A G in K; also let K L be drawn parallel to B G. K L will then be the Moment of the Ponderant D; and (by the last Article) it will be as B G the Moment of the Ponderant C in the point B, to L K the Moment of the Ponderant D in the point E so A B to A L. On the other side of the center of the Scale, let A N be taken equal to A L; and to the point N let there be applyed the Ponderant O, having to the Ponderant C the proportion of A B to A N. I say the Ponderants in B and N will be equally poised. For the proportion of the Moment of the Ponderant O in the point N, to the Moment of the Ponderant C in the point B, is (by the 5th Article) co¯pounded of the proportions of the waight O to the waight B, & of the distance (from the center of the Scale) A N or A L to the dista¯ce (fro¯ the center of the Scale) A B. But seeing we have supposed, that the distance A B to the distance A N, is in reciprocal proportion of the Waight O to the waight C, the proportion of the Moment of the Ponderant O in the point N, to the Moment of the Ponderant C in the point B, will be compounded of the proportions of A B to A N, and of A N to A B. Wherefore, setting in order A B, A N, A B, the Moment of O to the Moment of C will be as the first to the last, that is, as A B to A B. Their Moments therefore are equal; and consequently the Plain which passes through A, will (by the fifth Definition) be a Plain of Equiponderation. Wherefore they will be equally poised; as was to be proved.
Now the converse of this is manifest. For if there be Equiponderation, and the proportion of the Waights and Distances be not reciprocal, then both the Waights will alwayes have the same Moments, although one of them have more waight added to it, or its distance changed.
Corollary. When Ponderants are of the same Species, and their Moments be equal; their Magnitudes and Distances from the center of the Scale will be reciprocally proportional. For in Homogeneous Bodies, it is as Waight to Waight, so Magnitude to Magaltude.
8 If to the whole length of the Beam there be applyed a Parallelogram, or a Parallelopipedum, or a Prisma, or a Cylinder, or the Superficies of a Cylinder, ot of a Sphere, or of any portion of a Sphere or Prisma; the parts of any of them cut off with plains parallel to the base, will have their Moments in the same proportion with the parts of a Triangle which has its Vertex in the center of the Scale, and for one of its sides the Beam it self, which parts are cut off by Plains parallel to the base.
First, let the rectangled Parallelogram A B C D (in the 4th figure) be applyed to the whole length of the Beam A B; and producing C B howsoever to E, let the Triangle A B E be described. Let now any part of the Parallelogram, as A F, be cut off by the plain F G, parallel to the base C B; and let F G be produced to A E in the point H. I say the Moment of the whole A B C D to the Moment of its part A F, is as the Triangle A B E to the Triangle A G H, that is, in proportion duplicate to that of the distances from the center of the Scale.
For, the Parallelogram A B C D being divided into equal parts infinite in number, by straight lines drawn parallel to the base; and supposing the Moment of the straight line C B to be B E; the Moment of the straight line F G, will (by the 7th Article) be G H; and the Moments of all the straight lines of that Parallelogram, will be so many straight lines in the Triangle A B E drawn parallel to the base B E; all which parallels together taken are the Moment of the whole Parallelogram A B C D; and the same parallels do also constitute the superficies of the Triangle A B E. Wherefore the Moment of the Parallelogram A B C D, is the Triangle A B E; and for the same reason, the Moment of the Parallelogram A F, is the Triangle A G H; and therefore the Moment of the whole Parallelogram, to the Moment of a Parallelogram which is part of the same, is as the Triangle A B E, to the Triangle A G H, or in proportion duplicate to that of the Beams to which they are applyed. And what is here demonstrated in the case of a Parallelogram, may be understood to serve for that of a Cylinder, and of a Prisma, and their Superficies; as also for the Superficies of a Sphere, of an Hemisphere, or any portion of a Sphere, (for the parts of the Superficies of a Sphere, have the same proportion with that of the parts of the Axis cut off by the same parallels by which the parts of the Superficies are cut off, as Archimedes has demonstrated); and therefore when the parts of any of these figures are equal and at equal distances from the Center of the Scale, their Moments also are equal, in the same manner as they are in Parallelograms.
Secondly, let the Parallelogram A K I B not be rectangled; the straight line I B wil nevertheless press the point B perpendicularly in the straight line B E; & the straight line L G wil press the point G perpendicularly in the straight line G H; and all the rest of the straight lines which are parallel to I B will do the like. Whatsoever therefore the Moment be which is assigned to the straight line I B, as here (for example) it is supposed to be B E, if A E be drawn, the Moment of the whole Parallelogram A I will be the Triangle A B E; and the Moment of the part A L will be the Triangle A G H. Wherefore the Moment of any Ponderant, which has its sides equally applyed to the Beam, (whether they be applyed perpendicularly or obliquely) will be always to the Moment of a part of the same, in such proportion, as the whole Triangle has to a part of the same cut off by a plain which is parallel to the base.
9 The Center of Equiponderation of any figure which is deficient according to commensurable proportions of the altitude and base diminished, and whose complete figure is either a Parallelogram, or a Cylinder, or a Parallelopipedum, divides the Axis, so, that the part next the Vertex, to the other part, is as the complete figure to the deficient figure.
For let C I A P E (in the 5th figure) be a deficient figure, whose Axis is A B, and whose complete figure is C D F E; and let the Axis A B be so divided in Z, that A Z be to Z B as C D F E is to C I A P E. I say the center of Equiponderation of the figure C I A P E will be in the point Z.
First, that the Center of Equiponderation of the figure C I AP E is somewhere in the Axis A B, is manifest of it self; and therefore A B is a Diameter of Equiponderation. Let A E be drawn, and let B E be put for the Moment of the straight line C E; the Triangle A B E will therefore (by the 3d Article) be the Moment of the complete figure C D F E. Let the Axis A B be equally divided in L, and let G L H be drawn parallel and equal to the straight line C E, cutting the crooked line C I A P E in I and P, and the straight lines A C and A E in K and M. Moreover, let Z O be drawn parallel to the same C E; and let it be, as L G to L I, so L M to another L N; and let the same be done in all the rest of the straight lines possible, parallel to the base; and through all the points N, let the line A N E be drawn; the three-sided figure A N E B will therefore be the Moment of the figure C I A P E. Now the Triangle A B E is (by the 9th Article of the 17th Chapter) to the three-sided figure A N E B, as A B C D + A I C B is to A I C B twice taken, that is, as C D F E + C I A P E is to C I A P E twice taken. But as C I A P E is to C D F E, that is, as the waight of the deficient figure, is to the waight of the complete figure, so is C I A P E twice taken, to C D F E twice taken. Wherefore, setting in order C D F E + C I A P E. 2 C I A P E. 2 C D F E; the proportion of C D F E + C I A P E to C D F E twice taken, will be compounded of the proportion of C D F E + C I A P E to C I A P E twice taken, that is, of the proportion of the Triangle A B E to the threesided figure A N E B, that is, of the Moment of the complete figure to the Moment of the deficient figure, and of the proportion of C I A P E twice taken, to C D F E twice taken, that is, to the proportion reciprocally taken of the waight of the deficient figure to the waight of the complete figure.
Again, seeing by supposition A Z. Z B:: C D F E. C I A P E are proportionals; A B. A Z:: C D F E + C I A P E. C D F E will also (by co¯pounding) be proportionals. And seeing A L is the half of A B, A L. A Z:: C D F E + C I A P E. 2 C D F E will also be proportionals. But the proportion of C D F E + C I A P E to 2 C D F E is compounded (as was but now shewn) of the proportions of Moment to Moment &c. and therefore the proportion of A L to A Z is compounded of the proportion of the Moment of the complete figure C D F E to the Moment of the deficient figure C I A P E, and of the proportion of the waight of the deficient figure C I AP E, to the waight of the complete figure C D F E; But the proportion of A L to A Z is compounded of the proportions of A L to B Z and of B Z to A Z. Now the proportion of B Z to A Z is the proportion of the Waights reciprocally taken, that is to say, of the waight C I A P F to the waight C D F E. Therefore the remayning proportion of A L to B Z, that is, of L B to B Z is the proportion of the Moment of the waight C D F E to the Moment of the waight C I A P E. But the proportion of A L to B Z is compounded of the proportions of A L to A Z and of A Z to Z B; of which proportions that of A Z to Z B is the proportion of the waight C D F E to the waight C I A P E. Wherefore (by the 5th Article of this Chapter) the remayning proportion of A L to A Z is the proportion of the distances of the points Z and L from the center of the Scale, which is A. And therefore (by the 6th Article) the waight C I A P E shall hang from O in the straight line O Z. So that O Z is one Diameter of Equiponderation of the waight C I A P E. But the straight line A B is the other Diameter of Equiponderation of the same waight C I A P E. Wherefore (by the 7th Definition) the point Z is the center of the same Equiponderation; which point (by construction) divides the axis so, that the part A Z which is the part next the vertex, is to the other part Z B, as the complete figure C D F E is to the deficient figure C I A P E; which is that which was to be demonstrated.
Corollary. The Center of Equiponderation of any of those plain three-sided figures, which are compared with their complete figures in the Table of the third Article of the 17th Chapter, is to be found in the same Table, by taking the Denominator of the fraction for the part of the axis cut off next the vertex, and the Numerator for the other part next the base. For example, if it be required to find the Center of Equiponderation of the second threesided figure of foure Meanes, there is in the concourse of the second columne with the row of three-sided figures of four Meanes this fraction /7, which signifies that that figure is to its parallelogra¯ or compleat figure as 5/7 to Unity, that is, as 5/7 to 7/7, or as 5 to 7; and therefore the Center of Equiponderation of that figure, divides the axis, so, that the part next the vertex is to the other part as 7 to 5.
2 Corallary. The Center of Equiponderation of any of the Solids of those figures which are contained in the Table of the 8th Article of the same 17th Chapter, is exhibited in the same Table. For example, if the Center of Equiponderation of a Cone be sought for; the Cone will be found to be ⅓ of its Cylinder; and therefore the Center of its Equiponderation will so divide the axis, that the part next the vertex, to the other part, will be as 3 to 1. Also the Solid of a three-sided figure of one Meane, that is, a parabolical Solid, seeing it is 2/4, that is ½ of its Cylinder, will have its Center of Equiponderation in that point, which divides the axis, so, that the part towards the vertex be double to the part towards the base.
10 The Diameter of Equiponderation of the Complement of the half of any of those figues which are contained in the Table of the 3d article of the 17th Chapter, divides that line which is drawne through the Vertex parallel and equall to the base, so, that the part next the Vertex, will be to the other part, as the Complete figure to the Complement.
For let A I C B (in the same 5 fig.) be the halfe of a Parabola, or of any other of those three-sided figures which are in the Table of the 3d article of the 17th Chap whose Axis is A B, and base B C; having A D drawn from the Vertex, equall and parallel to the base B C; and whose complete figure is the parallelogramme A B C D. Let I Q be drawne, at any distance from the side C D, but parallel to it; and let A D be the altitude of the Complement A I C D, and Q I a line ordinately applyed in it. Wherefore the altitude A L in the deficient figure A I C B, is equal to Q I the line ordinately applyed in its Complement; and contrarily, L I the line ordinately applyed in the figure A I C B, is equall to the altitude A Q in its Complement; and so in all the rest of the ordinate lines and altitudes, the mutation is such, that that line which is ordinately applyed in the figure, is the altitude of its Complement. And therefore the proportion of the altitudes decreasing, to that of the ordinate lines decreasing, being multiplicate according to any number in the deficient figure, is submultiplicate according to the same number in its Complement. For example, if A I C B be a Parabola, seeing the proportion of A B to A L is duplicate to that of B C to L I, the proportion of A D to A Q in the Complement A I C D (which is the same with that of B C to L I) will be subduplicate to that of C D to Q I (which is the same with that of A B to A L); and consequently, in a Parabola, the Complement will be to the Parallelogramme as 1 to 3; in a three-sided figure of two Meanes, as 1 to 4; in a three-sided figure of three Meanes, as 1 to 5, &c. But all the ordinate lines together in A I C D are its moment; and all the ordinate lines in A I C B are its moment. Wherefore the moments of the Complements of the halves of Deficient figures in the Table of the 3d article of the 17th Chap. being compared, are as the Deficient figures themselves; and therefore the Diameter of Equiponderation will divide the straight line A D in such proportion, that the part next the Vertex be to the other part, as the complete figure A B C D is to the Complement A I C D.
Coroll. The diameter of Equiponderation of these halves, may be found by the Table of the d article of the 17th Chapter in this manner. Let there be propounded any deficient figure, namely the second three-sided figure of two Meanes. This figure is to the complete figure as ⅗ to 1, that is as 3 to 5. Wherefore the Complement to the same complete figure is as 2 to 5; and therefore the diameter of Equiponderation of this Complement will cut the straight line drawne from the Vertex parallel to the base, so, that the part next the Vertex will be to the other part as 5 to 2. And in like manner, any other of the said three-sided figures being propounded, if the numerator of its fraction (found out in the Table) be taken from the denominator, the straight line drawn from the Vertex is to be divided, so, that the part next the Vertex be to the other part, as the denominator is to the remainder which that substraction leaves.
11 The center of Equiponderation of the halfe of any of those crooked-lined figures which are in the first row of the Table of the 3d article of the 17th chapter, is in that straight line, which being parallel to the Axis, divides the base according to the numbers of the fraction next below it in the second row, so, that the Numerator be answerable to that part which is towards the Axis.
For example, let the first figure of three Means be taken, whose half is A B C D (in the 6th figure), and let the rectangle A B E D be completed. The Complement therefore will be B C D E. And seeing A B E D is to the figure A B C D (by the Table) as 5 to 4, the same A B E D will be to the Complement B C D E as 5 to 1. Wherefore if F G be drawn parallel to the base D A, cutting the axis, so, that A G be to G B as 4 to 5, the center of Equiponderation of the figure A B C D, will (by the precedent article) be somewhere in the same F G. Again, seeing (by the same article) the complete figure A B E D, is to the Complement B C D E as 5 to 1, therefore if B E and A D be divided in H and I as 5 to 1, the center of Equiponderation of the Complement B C D E will be somewhere in the straight line which connects H and I. Let now the straight line L K be drawn through M the center of the complete figure, parallel to the base; and the straight line N O, through the same center M, perpendicular to it; and let the straight lines L K and F G cut the straight line H I in P and Q. Let P R be taken quadruple to P Q; and let R M be drawn and produced to F G in S. R M therefore will be to M S as 4 to 1, that is, as the figure A B C D to its Complement B C D E. Wherefore seeing M is the center of the Complete figure A B E D, and the distances of R and S from the center M be in proportion reciprocall to that of the waight of the Complement B C D E to the waight of the figure A B C D, R and S will either be the centers of Equiponderation of their own figures, or those centers will be in some other points of the diameters of Equiponderation H I and F G. But this last is impossible. For no other straight line can be drawn through the point M terminating in the straight lines H I and F G, and retaining the proportion of M R to M S, that is, of the figure A B C D to its complement B C D E. The center therefore of Equiponderation of the figure A B C D is in the point S. Now seeing P M hath the same proportion to Q S which R P hath to R Q, Q S will be 5 of those parts of which P M is 4, that is, of which I N is 4. But I N or P M is 2 of those parts of which E B or F G is 6; and therefore if it be, as 4 to 5, so 2 to a fourth, that fourth will be 2½. Wherefore Q S is 2½ of those parts of which F G is 6. But F Q is 1; and therefore F S is 3½. Wherefore the remayning part G S is 2½. So that F G is so divided in S, that the part towards the Axis, is in proportion to the other part as 2½ to 3½, that is, as 5 to 7; which answereth to the fraction 5/7 in the second row, next under the fraction ⅘ in the first row. Wherefore drawing S T parallel to the Axis, the base wil be divided in like manner.
By this Method it is manifest, that the base of a Semiparabola will be divided into 3 and 5; and the base of the first three-sided figure of two Means, into 4 and 6; and of the first three-sided figure of four Means, into 6 and 8. The fractions therefore of the second row denote the proportions into which the bases of the figures of the first row are divided by the diameters of Equiponderation. But the first row begins one place higher then the second row.
12 The center of Equiponderation of the half of any of the figures in the second row of the same Table of the 3d article of the 17th Chapter, is in a straight line parallel to the Axis, and dividing the base according to the nu¯bers of the fraction in the fourth row, two places lower, so, as that the Numerator be answerable to that part which is next the Axis.
Let the half of the second three-sided figure of two Means be taken; and let it be A B C D (in the 7th Figure); whose complement is B C D E, and the rectangle completed A B E D. Let this rectangle be divided by the two straight lines L K & N O, cutting one another in the center M at right angles; and because A B E D is to A B C D as 5 to 3, let A B be divided in G, so, that A G to B G be as 3 to 5; and let F G be drawn parallel to the base. Also because A B E D is (by the 9th article) to B C D E as 5 to 2, let B E be divided in the point I, so, that B I be to I E as 5 to 2; and let I H be drawn parallel to the Axis, cutting L K and F G in P and Q. Let now P R be so taken, that it be to P Q as 3 to 2, and let R M be drawn and produced to F G in S. Seeing therefore R P is to R Q, that is, R M to M S, as A B C D is to its complement B C D E, and the centers of Equiponderation of A B C D and B C D E are in the straight lines F G and H I, and the center of Equiponderation of them both together in the point M; R will be the center of the Complement B C D E, and S the center of the Figure A B C D. And seeing P M, that is I N, is to Q S, as R P is to R Q; and I N, or P M is 3 of those parts, of which B E, that is, F G is 14; therefore Q S is 5 of the same parts; and E I, that is F Q, 4; and F S, 9; and G S, 5. Wherefore the straight line S T being drawn parallel to the Axis, will divide the base A D into 5 and 9. But the fraction 5/9 is found in the fourth row of the Table, two places below the fraction 9/5 in the second row.
By the same method, if in the same second row, there be taken the second three-sided Figure of three Meanes, the center of Equiponderation of the half of it, will be found to be in a straight line parallel to the Axis, dividing the base according to the numbers of the fraction 6/10, two places below in the fourth row. And the same way serves for all the rest of the Figures in the second row. In like manner, the center of Equiponderation of the third three-sided Figure of three Means, will be found to be in a straight line parallel to the Axis, dividing the base, so, that the part next the Axis, be to the other part, as 7 to 13, &c.
Coroll. The Centers of Equiponderation of the halves of the said Figures are known, seeing they are in the intersection of the straight lines S T and F G, which are both known.
13 The center of Equiponderation of the half of any of the Figures, which (in the Table of the 3d Article of the 17th Chap.) are compared with their Parallelograms, being known; the center of Equiponderation of the excess of the same Figure above its triangle, is also known.
For example, let the Semiparabola A B C D (in the 8th Fig.) be taken; whose Axis is A B; whose complete Figure is A B E D; and whose excess above its triangle is B C D B. Its center of Equiponderation may be found out in this manner. Let F G be drawn parallel to the base, so, that A F be a third part of the Axis; and let H I be drawn parallel to the Axis, so, that A H be a third part of the base. This being done, the center of Equiponderation of the triangle A B D, will be I. Again, let K L be drawn parallel to the base, so, that A K be to A B as 2 to 5; and M N parallel to the Axis, so, that A M be to A D as 3 to 8; and let M N terminate in the straight line K L. The center therefore of Equiponderation of the Parabola A B C D is N; and therefore we have the centers of Equiponderation of the Semiparabola A B C D, and of its part the triangle A B D. That we may now finde the Center of Equiponderation of the remayning part B C D B, let I N be drawn and produced to O, so, that N O be triple to I N; and O will be the center sought for. For seeing the waight of A B D, to the waight of B C D B is in proportion reciprocall to that of the straight line N O to the straight line I N; and N is the center of the whole, and I the center of the triangle A B D; O will be the center of the remaining part, namely, of the figure B D C B; which was to be found.
Coroll. The Center of Equiponderation of the figure B D C B, is in the concourse of two straight lines, whereof one is parallel to the base, and divides the Axis, so, that the part next the base be ⅖ or 6/15 of the whole Axis; the other is parallel to the Axis, and so divides the base, that the part towards the Axis be ½ or 12/24 of the whole base. For drawing O P parallel to the base, it will be as I N to N O, so F K to K P, that is, so 1 to 3, or 5 to 15. But A F is 5/15 or ⅓ of the whole A B; and A K is 6/15 or ⅖; and F K /15; and K P 3/15; and therefore A P is 9/15 of the Axis A B. Also A H is ⅓ or 8/24; and A M ⅜ or 9/24 of the whole base; and therefore O Q being drawn parallel to the Axis, M Q (which is triple to H M) will be 3/24. Wherefore A Q is 12/24 or ½ of the base A D.
The excesses of the rest of the three-sided figures in the first row of the Table of the 3d article of the 17th Chapter, have their centers of Equiponderation in two straight lines which divide the Axis and base according to those fractions, which adde 4 to the numerators of the fractions of a Parabola 9/15 and 12/24; and 6 to the denominators, in this manner,
In a Parabola, The Axis 9/15, The Base 12/24
In the first three-sided figure, The Axis 13/21, The Base 16/30
In the second three-sided figure, The Axis 17/27, The Base 20/36 &c.
And by the same method, any man (if it be worth the paines) may find out the centers of Equiponderation of the excesses above their triangles of the rest of the figures in the second & third row, &c.
14 The center of Equiponderation of the Sector of a Sphere (that is, of a figure compounded of a right Cone whose Vertex is the center of the Sphere, and the portion of the Sphere whose base is the same with that of the Cone), divides the straight line which is made of the Axis of the Cone and halfe the Axis of the portion together taken, so, that the part next the Vertex be triple to the other part, or to the whole straight line, as 3 to 4.
For let A B C (in the 9th fig.) be the Sector of a Sphere, whose Vertex is the ceter of the Sphere A; whose Axis is A D; and the circle upon B C is the common base of the portion of the Sphere and of the Cone whose Vertex is A; the Axis of which portion is E D, and the halfe thereof F D; and the Axis of the Cone, A E. Lastly let A G be ¾ of the straight line A F. I say G is the center of Equiponderation of the Sector A B C.
Let the straight line F H be drawne of any length, making right angles with A F at F; and drawing the straight line A H, let the triangle A F H be made. Then upon the same center A let any arch I K be drawne, cutting A D in L; and its chord, cutting A D in M; and dividing M L equally in N, let N O be drawne parallel to the straight line F H, and meeting with the straight line A H in O.
Seeing now B D C is the Spherical Superficies of the portion cut off with a plain passing through B C, and cutting the Axis at right angles; and seeing F H divides E D the Axis of the portion into two equal parts in F; the center of Equiponderation of the Superficies B D C will be in F (by the 8th article); and for the same reason the center of Equiponderation of the Superficies I L K (K being in the straight line A C) will be in N. And in like manner, if there were drawne between the center of the Sphere A and the outermost Spherical Superficies of the Sector, arches infinite in number, the centers of Equiponderation of the Sphericall Superficies in which those arches are,, would be found to be in that part of the Axis, which is intercepted between the Superficies it selfe and a plaine passing along by the chord of the arch, and cutting the Axis in the middle at right angles.
Let it now be supposed that the moment of the outermost sphericall Superficies B D C is F H. Seeing therefore the Superficies B D C is to the Superficies I L K in proportion duplicate to that of the arch B D C to the arch I L K, that is, of B E to I M, that is, of F H to N O; let it be as F H to N O, so N O to another N P; and again, as N O to N P, so N P to another N Q; and let this be done in all the straight lines parallel to the base F H that can possibly be drawn between the base and the vertex of the triangle A F H. If then through all the points Q there be drawn the crooked line A Q H, the figure A F H Q A will be the complement of the first three-sied figure of two Meanes; and the same will also be the moment of all the Sphericall Superficies of which the Solid Sector A B C D is compounded; and by consequent, the moment of the Sector it selfe. Let now F H be understood to be the semidiameter of the base of a right Cone, whose side is A H, and Axis A F. Wherfore seeing the bases of the Cones which passe through F and N and the rest of the points of the Axis, are in proportion duplicate to that of the straight lines F H and N O, &c. the moment of all the bases together, that is, of the whole Cone, will be the figure it self A F H Q A; and therefore the center of Equiponderation of the Cone A F H is the same with that of the solid Sector. Wherefore seeing A G is ¾ of the Axis A F, the center of Equiponderation of the Cone A F H is in G; and therefore the center of the solid Sector is in G also, and divides the part A F of the Axis, so, that A G is triple to G F; that is, A G is to A F as 3 to 4; which was to be demonstrated.
Note, that when the Sector is a Hemisphere, the Axis of the Cone vanisheth into that point which is the center of the Sphere; and therefore it addeth nothing to half the Axis of the portion. Wherefore, if in the Axis of the Hemisphere, there be taken from the center, ¾ of halfe the Axis, that is, 3/ of the Semidiameter of the Sphere, there will be the center of Equiponderation of the Hemisphere.
CHAP. XXIV. OF REFRACTION AND REFLECTION.
1 DEFINITIONS.
2 In perpendicular Motion there is no Refraction.
3 Things thrown out of a thinner into a thicker Medium, are so refracted, that the Angle Refracted is greater then the Angle of Inclination.
4 Endeavour which from one point tendeth every way, will be so Refracted, at that the sine of the Angle Refracted, will be to the sine of the Angle of Inclination, as the Density of the first Medium is to the Density of the second Medium, reciprocally taken.
5 The sine of the Refracted Angle in one Inclination is to the sine of the Refracted Angle in another Inclination, as the sine of the Angle of that Inclination is to the sine of the Angle of this Inclination.
6 If two lines of Incidence, having equal Inclination, be the one in a thinner, the other in a thicker Medium, the sine of the angle of Inclination will be a Mean proportional between the two sines of the Refracted angles.
7 If the angle of Inclination be semirect, and the line of Inclination be in the thicker Medium, and the proportion of their Densities be the same with that of the Diagonal to the side of a Square, and the separating Superficies be plain, the Refracted line will be in the separating Superficies.
8 If a Body be carried in a straight line upon another Body, and do not penetrate the same, but be reflected from it, the angle of Reflexion will be equal to the Angle of Incidence.
9 The same happens in the generation of Motion in the line of Incidence.
1
DEFINITIONS.
1 REFRACTION, IS the breaking of that straight Line, in which a Body is moved, or its Action would proceed in one and the same Medium, into two straight lines, by reason of the different natures of the two Mediums.
2 The former of these is called the Line of Incidence; the later the Refracted Line.
3 The Point of Refraction, is the common point of the Line of Incidence and of the Refracted Line.
4 The Refracting Superficies, which also is the Separating Superficies of the two Mediums, is that in which is the point of Refraction.
5 The Angle Refracted, is that which the Refracted Line makes in the point of Refraction, with that Line which from the same point is drawn perpendicular to the separating Superficies in a different Medium.
6 The Angle of Refraction, is that which the Refracted line makes with the Line of Incidence produced.
7 The Angle of Inclination, is that which the Line of Incidence makes with that Line which from the point of Refraction is drawn perpendicular to the separating Superficies.
8 The Angle of Incidence, is the Complement to a right Angle of the Angle of Inclination.
And so, (in the first Figure) the Refraction is made in A B F. The Refracted Line is B F. The Line of Incidence is A B. The Point of Incidence, and of Refraction is B. The Refracting or Separating Superficies is D B E. The Line of Incidence produced directly is A B C The Perpendicular to the separating Superficies is B H. The Angle of Refraction is C B F. The Angle Refracted is H B F. The Angle of Inclination is A B G or H B C. The Angle of Incidence is A B D.
9 Moreover the Thinner Medium, is understood to be that in which there is less resistance to Motion or to the generation of Motion; & the Thicker, that wherin there is greater resistance.
10 And that Medium in which there is equal resistance every where, is a Homogeneous Medium. All other Mediums are Heterogeneous.
2 If a Body pass, or there be generation of Motion, from one Medium to another of different Density, in a line perpendicular to the Separating Superficies; there will be no Refraction.
For seeing on every side of the perdendicular all things in the Mediums are supposed to be like and equal; if the Motion it self be supposed to be perpendicular, the Inclinations also will be equal, or rather none at all; and therefore there can be no cause, from which Refraction may be inferred to be on one side of the perpendicular, which wil not co¯clude the same Refraction to be on the other side. Which being so, Refraction on one side will destroy Refraction on the other side; and consequently, either the Refracted line will be every where, (which is absurd), or there will be no Refracted line at all; which was to be demonstrated.
Corol. It is manifest from hence, that the cause of Refraction consisteth onely in the obliquity of the line of Incidence, whether the Incident Body penetrate both the Mediums, or without penetrating, propagate motion by Pressure onely.
3 If a Body, without any change of situation of its internal parts, as a stone, be moved obliquely out of the thinner Medium, and proceed penetrating the thicker Medium; and the thicker Medium be such, as that its internal parts being moved, restore themselves to their former situation; the angle Refracted will be greater then the angle of Inclination.
For let D B E (in the same first figure) be the separating Superficies of two Mediums; and let a Body, as a stone thrown, be understood to be moved as is supposed in the straight line A B C; and let A B be in the thinner Medium, as in the Aire; and B C in the thicker, as in the Water. I say the stone, wch being thrown, is moved in the line A B, will not proceed in the line B C, but in some other line, namely that, with which the perpendicular B H makes the Refracted angle H B F greater then the angle of Inclination H B C.
For seeing the stone coming from A, and falling upon B, makes that which is at B proceed towards H, and that the like is done in all the straight lines which are parallel to B H; and seeing the parts moved restore themselves by contrary motion in the same line; there will be contrary motion generated in H B, and in all the straight lines which are parallel to it. Wherefore the motion of the stone will be made by the concourse of the motions in A G, that is, in D B, and in G B, that is, in B H, and lastly, in H B, that is, by the concourse of three motions. But by the concourse of the motions in A G and B H, the stone will be carried to C; and therefore by adding the motion in H B, it will be carried higher in some other line, as in B F, and make the angle H B F greater then the angle H B C.
And from hence may be derived the cause, why Bodies which are thrown in a very oblique line, if either they be any thing flat, or be thrown with great force, will when they fall upon the water, be cast up again from the water into the aire.
For let A B (in the 2d figure) be the superficies of the water; into which from the point C, let a stone be thrown in the straight line C A, making with the line B A produced a very little angle C A D; and producing B A indefinitely to D, let C D be drawn perpendicular to it, and A E parallel to C D. The stone therefore will be moved in C A by the concourse of two motions in C D and D A, whose velocities are as the lines themselves C D and D A. And from the motion in C D and all its parallels downwards, as soon as the stone falls upon A, there will be Reaction upwards, because the water restores it self to its former situation. If now the stone be thrown with sufficient obliquity, that is, if the straight line C D be short enough, that is, if the endeavour of the stone downwards be less then the Reaction of the water upwards, that is, less then the endeavour it hath from its own gravity, (for that may be), the stone will (by reason of the excess of the endeavour which the water hath to restore it self, above that which the stone hath downwards) be raised again above the Superficies A B, and be carried higher, being reflected in a line which goes higher, as the line A G.
4 If from a point, whatsoever the Medium be, Endeavour be propagated every way into all the parts of that Medium; and to the same Endeavour there be obliquely opposed another Medium of a different nature, that is, either thinner or thicker; that Endeavour will be so refracted, that the sine of the angle Refracted, to the sine of the angle of Inclination, will be as the density of the first Medium to the density of the second Medium, reciprocally taken.
First, let a Body be in the thinner Medium in A (Figure 3d.); and let it be understood to have endeavour every way, and consequently that its endeavour proceed in the lines A B and A b; to which let B b the superficies of the thicker Medium be obliquely opposed in B and b, so that A B and A b be equal; and let the straight line B b be produced both wayes. From the points B and b let the perpendiculars B C and b c be drawn; and upon the centers B and b, and at the equal distances B A and b A, let the Circles A C and A c be described, cutting B C and b c in C and c, and the same C B and c b produced in D and d, as also A B and A b produced in E and e. Then from the point A to the straight lines B C and b c let the perpendiculars A F and A f be drawn. A F therefore will be the sine of the angle of Inclination of the straight line A B, and A f the sine of the angle of Inclination of the straight line A h, which two Inclinations are by construction made equal. I say, as the density of the Medium in which are B C and b c, is to the density of the Medium in which are B D and b d, so is the sine of the angle Refracted, to the sine of the angle of Inclination.
Let the straight line F G be drawn parallel to the straight line A B, meeting with the straight line b B produced in G.
Seeing therefore A F and B G are also parallels, they will be equal; and consequently, the endeavour in A F is propagated in the same time, in which the endeavour in B G would be propagated if the Medium were of the same density. But because B G is in a thicker Medium, that is, in a Medium which resists the endeavour more then the Medium in which A F is, the endeavour will be propagated less in B G then in A F, according to the proportion which the density of the Medium in which A F is, hath to the density of the Medium in which B G is. Let therefore the density of the Medium in which B G is, be to the density of the Medium in which A F is, as B G is to B H; and let the measure of the time be the Radius of the Circle. Let H I be drawn parallel to B D, meeting with the circumference in I; and from the point I let I K be drawn perpendicular to B D; which being done, B H and I K will be equal; and I K will be to A F, as the density of the Medium in which is A F, is to the density of the Medium in which is I K. Seeing therefore in the time A B (which is the Radius of the Circle) the endeavour is propagated in A F in the thinner Medium, it will be propagated in the same time, that is, in the time B I in the thicker Medium from K to I. Therefore B I is the Refracted line of the line of Incidence A B; and I K is the sine of the angle Refracted; and A F, the sine of the angle of Inclination. Wherefore seeing I K is to A F, as the density of the Medium in which is A F to the density of the Medium in which is I K; it will be as the density of the Medium in which is A F, (or B C) to the density of the Medium in which is I K (or B D), so the sine of the angle Refracted to the sine of the angle of Inclination. And by the same reason it may be shewn, that as the density of the thinner Medium is to the density of the thicker Medium, so will K I the sine of the angle Refracted be to A F the sine of the Angle of Inclination.
Secondly, let the Body which endeavoureth every way, be in the thicker Medium at I. If therefore both the Mediums were of the same density, the endeavour of the Body in I B would tend directly to L; and the sine of the angle of Inclination L M would be equal to I K or B H. But because the density of the Medium in which is IK, to the density of the Medium in which is L M, is as BH to B G, that is, to A F, the endeavour will be propagated further in the Medium in which L M is, then in the Medium in which I K is, in the proportion of density to density, that is, of M L to A F. Wherefore B A being drawn, the angle Refracted will be C B A, and its sine A F. But L M is the sine of the angle of Inclination; and therefore again, as the density of one Medium is to the density of the different Medium, so reciprocally is the sine of the angle Refracted to the sine of the angle of Inclination, which was to be demonstrated.
In this Demonstration, I have made the separating Superficies B b plain by construction. But though it were concave or convex, the Theoreme would nevertheless be true. For the Refraction being made in the point B of the plain separating Superficies, if a crooked line, as P Q be drawn, touching the separating line in the point B; neither the Refracted line B I, nor the perpendicular B D will be altered; and the Refracted angle K B I, as also its sine K I will be still the same they were.
5 The sine of the angle Refracted in one Inclination, is to the sine of the angle Refracted in another Inclination, as the sine of the angle of that Inclination to the sine of the angle of this Inclination.
For seeing the sine of the Refracted angle is to the sine of the angle of Inclination, (whatsoever that Inclination be) as the density of one Medium, to the density of the other Medium; the proportion of the sine of the Refracted angle, to the sine of the angle of Inclination, will be compounded of the proportions of density to density, and of the sine of the angle of one Inclination to the sine of the angle of the other Inclination. But the proportions of the densities in the same Homogeneous Body, are supposed to be the same. Wherefore Refracted angles in different Inclinations, are as the sines of the angles of those Inclinations; which was to be demonstrated.
6 If two lines of Incidence having equal inclination, be the one in a thinner, the other in a thicker Medium; the sine of the angle of their Inclination, will be a mean proportional between the two sines of their angles Refracted.
For let the straight line AB (in the same 3d figure) have its Inclination in the thinner Medium, and be refracted in the thicker Medium in B I; and let E B have as much Inclination in the thicker Medium, and be refracted in the thinner Medium in B S; and let R S the sine of the angle Refracted be drawn. I say the straight lines R S, A F and I K are in continual proportion. For it is, as the density of the thicker Medium to the density of the thinner Medium, so R S to A F. But it is also, as the density of the same thicker Medium, to that of the same thinner Medium, so AF to IK. Wherefore R S. A F:: A F. I K are propoortionals; that is, R S, A F and I K are in continual proportion, and A F is the Mean proportional; which was to be proved.
7 If the angle of Inclination be semirect, and the line of Inclination be in the thicker Medium, and the proportion of the Densities be as that of a Diagonal to the side of its Square, and the separating Superficies be plain, the Refracted line will be in that separating Superficies.
For in the Circle A C (in the 4th figure) let the angle of Inclination A B C be an angle of 45 degrees. Let C B be produced to the Circumference in D; & let C E (the sine of the angle B C) be drawn to which, let B F be taken equal in the separating line B G. B C E F will therefore be a Parallelogram, & F E & B C, that is, F E and B G equal. Let AG be drawn, namely, the Diagonal of the Square whose side is B G; and it will be, as A G to E F, so B G to B F; & so (by supposition) the density of the Medium in which C is, to the density of the Medium in which D is; and so also the sine of the angle Refracted to the sine of the angle of Inclination. Drawing therefore F D, & from D the line D H perpendicular to A B produced, DH will be the sine of the angle of Inclination. And seeing the sine of the angle Refracted is to the sine of the angle of Inclination, as the density of the Medium in which is C, is to the density of the Medium in which is D, that is, (by supposition) as A G is to F E, that is, as D H is to B G; and seeing D H is the sine of the angle of Inclination, B G will therefore be the sine of the angle Refracted. Wherefore B G will be the Refracted line, and lye in the plain separating Superficies; which was to be demonstrated.
Coroll. It is therefore manifest, that when the Inclination is greater then 45 degrees, as also when it is less, provided the density be greater, it may happen that the Refraction will not enter the thinner Medium at all.
8 If a Body fall in a straight line upon another Body, and do not penetrate it, but be reflected from it, the angle of Reflexion will be equal to the angle of Incidence.
Let there be a Body at A (in the 5th figure), which falling with straight motion in the line A C upon another Body at C, passeth no further, but is reflected; and let the angle of Incidence be any angle, as A C D. Let the straight line C E be drawn, making with D C produced the angle E C F equall to the angle A C D; and let A D be drawn perpendicular to the straight line D F. Also in the same straight line D F let C G be taken equall to C D; and let the perpendicular G E be raised, cutting C E in E. This being done, the triangles A C D and E C G will be equall and like. Let C H be drawn equal and parallel to the straight line A D; and let H C be produced indefinitely to I. Lastly let E A be drawn, which will passe through H, and be parallel and equall to G D. I say the motion from A to C in the straight line of Incidence AC, will be reflected in the straight line C E.
For the motion from A to C is made by two coefficient or concurrent motions, the one in A H parallel to D G, the other in A D perpendicular to the same D G; of which two motions, that in A H workes nothing upon the Body A after it has been moved as farre as C, because (by supposition) it doth not passe the straight line D G; whereas the endeavour in A D, that is in H C, worketh further towards I. But seeing it doth onely presse and not penetrate, there will be reaction in H, which causeth motion from C towards H; and in the mean time the motion in H E remaines the same it was in A H; and therefore the Body will now be moved by the concourse of two motions in C H and H E, which are equall to the two motions it had formerly in A H and H C. Wherefore it will be carried on in C E. The angle therefore of Reflection will be E C G, equall (by construction) to the angle A C D; which was to be demonstrated.
Now when the Body is considered but as a point, it is all one, whether the Superficies or line in which the Reflection is made, be straight or crooked; for the point of Incidence and Reflexion C, is as well in the crooked line which toucheth D G in C, as in D G it selfe.
9 But if we suppose that not a Body be moved, but some Endeavour onely be propagated from A to C, the Demonstration will neverthelesse be the same. For all Endeavour is motion; and when it hath reached the Solid Body in C, it presseth it, and endeavoureth further in C I. Wherefore the reaction will proceed in C H; and the endeavour in C H concurring with the endeavour in H E, will generate the endeavour in C E, in the same manner as in the repercussion of Bodies moved.
If therefore Endeavour be propagated from any point to the concave Superficies of a Spherical Body, the Reflected line with the circumference of a great circle in the same Sphere, will make an angle equall to the angle of Incidence.
For if Endeavour be propagated from A (in the 6 fig.) to the circumference in B, and the center of the Sphere be C, and the line C B be drawne, as also the Tangent D B E; and lastly if the angle F B D be made equall to the angle A B E, the Reflexion will be made in the line B F, as hath been newly shewn. Wherefore the angles which the straight lines A B and F B make with the circumference, will also be equall. But it is here to be noted that if C B be produced howsoever to G, the endeavour in the line G B C will proceed onely from the perpendicular reaction in G B; and that therefore there will be no other endeavour in the point B towards the parts which are within the Sphere, besides that which tends towards the center.
And here I put an end to the third part of this Discourse; in which I have considered Motion and Magnitude by themselves in the abstract. The fourth and last part, concerning the Phaenomena of Nature, that is to say, concerning the Motions and Magnitudes of the Bodies which are parts of the World, reall and existent, is that which followes.
THE FOURTH PART, OF PHYSIQUES, OR THE PHAENOMENA OF NATURE.
CHAP. XXV. OF SENSE AND ANIMAL MOTION.
1 THE CONNEXION of what hath been said with that which followeth.
2 The investigation of the nature of Sense, and the Definition of Sense.
3 The Subject and Object of Sense.
4 The Organ of Sense.
5 All Bodies are not indued with Sense.
6 But one Phantasme at one and the same time.
7 Imagination, the Remayns of past Sense, (which also is Memory). Of Sleep.
8 How Phantasmes succeed one another.
9 Dreames, whence they proceed.
10 Of the Senses, their kindes, their Organs, and Phantasmes proper and common.
11 The Magnitude of Images, how and by what it is determined.
12 Pleasure, Pain, Appetite, and Aversion, what they are.
13 Deliberation and Will, what.
1 I Have (in the first Chapter) defined Philosophy to be Knowledge of Effects acquired by true Ratiocination, from knowledge first had of their Causes and Generation; and of such Causes or Generations as may be, from former knowledge of their Effects or Appearances. There are therefore two Methods of Philosophy, One from the Generation of things to their possible Effects, and the other from their Effects or Appearances to some possible Generation of the same. In the former of these, the Truth of the first Principles of our ratiocination (namely Definitions) is made and constituted by our selves, whilest we consent and agree about the Appellations of things. And this part I have finished in the foregoing Chapters; in which (if I am not deceived) I have affirmed nothing (saving the Definitions themselves) which hath not good coherence with the Definitions I have given; that is to say, which is not sufficiently demonstrated to all those that agree with me in the use of Words and Appellations; for whose sake onely I have written the same. I now enter upon the other part; which is the finding out by the Appearances or Effects of Nature which we know by Sense, some wayes and means by which they may be (I do not say, they are) generated. The Principles therefore, upon which the following discourse depends, are not such as we our selves make and pronounce in general terms, as Definitions; but such, as being placed in the things themselves by the Authour of Nature, are by us observed in them; and we make use of them in single and particular, not universal propositions. Nor do they impose upon us any necessity of constituting Theoremes; their use being onely (though not without such general Propositions as have been already demonstrated) to shew us the possibility of some production or generation. Seeing therefore the Science which is here taught, hath its Principles in the Appearances of Nature, and endeth in the attayning of some knowledge of Natural causes, I have given to this Part, the title of PHYSIQUES, or the PHAENOMENA of NATURE. Now such things as appear, or are shewn to us by Nature, we call Phaenomena or Appearances.
Of all the Phaenomena, or Appearances which are neer us, the most admirable is Apparition it self,; namely, that some Natural Bodies have in themselves the patterns almost of all things, and others of none at all. So that if the Appearances be the Principles by which we know all other things, we must needs acknowledge Sense to be the Principle by which we know those Principles; & that all the knowledge we have is derived from it. And as for the causes of Sense, we cannot begin our search of them from any other Phaenomenon then that of Sense it self. But you will say, by what Sense shall we take notice of Sense? I answer, by Sense it self, namely, by the Memory which for some time remains in us of things sensible, though they themselves pass away. For he that perceiues that he hath perceived, remembers.
In the first place therefore the causes of our Perception, that is, the causes of those Ideas and Phantasmes which are perpetually generated within us whilest we make use of our Senses, are to be enquired into; and in what manner their generation proceeds. To help which Inquisition, we may observe first of all, that our Phantasmes or Ideas are not alwayes the same; but that new ones appear to us, and old ones vanish, according as we apply our Organs of Sense, now to one Object, now to another. Wherefore they are generated, and perish. And from hence it is manifest, that they are some change or mutation in the Sentient.
2 Now that all Mutation or Alteration is Motion, or Endeavour (and Endeavour also is Motion) in the internal parts of the thing that is altered, hath been proved (in the 9th Article of the 8th Chapter) from this, that whilest even the least parts of any Body remain in the same situation in respect of one another, it cannot be said that any alteration (unless perhaps that the whole Body together hath been moved) hath hapned to it; but that it both appeareth and is the same it appeared & was before. Sense therefore in the Sentient, can be nothing else but motion in some of the internal parts of the Sentient; and the parts so moved, are parts of the Organs of Sense. For the parts of our Body by which we perceive any thing, are those we commonly call the Organs of Sense. And so we find what is the Subject of our Sense, namely, that in which are the Phantasmes; and partly also we have discovered the nature of Sense, namely, that it is some internal Motion in the Sentient.
I have shewn besides (in the 8th Chap. at the 7th Article) that no Motion is generated but by a Body contiguous and Moved. From whence it is manifest, that the immediate cause of Sense or Perception consists in this, that the first Organ of Sense is touched and pressed. For when the uttermost part of the Organ is pressed, it no sooner yeilds, but the part next within it, is pressed also; and in this manner, the pressure or Motion is propagated through all the parts of the Organ to the innermost. And thus also the pressure of the uttermost part proceeds from the pressure of some more remote Body, and so continually, till we come to that from which, as from its fountain we derive the Phantasme or Idea that is made in us by our Sense. And this, whatsoever it be, is that we commonly call the Object. Sense therefore is some internal Motion in the Sentient, generated by some internal Motion of the parts of the Object, and propagated through all the Media to the innermost part of the Organ. By which words I have almost defined what Sense is.
Moreover, I have shewn (in the 2d Article of the 15 Chapter) that all Resistance is Endeavour opposite to another Endeavour, that is to say, Reaction. Seeing therefore there is in the whole Organ by reason of its own internal natural Motion; some Resistance or Reaction against the Motion which is propagated from the Object to the innermost part of the Organ, there is also in the same Organ an Endeavour opposite to the Endeavour which proceeds from the Object; so that when that Endeavour inwards is the last action in the act of Sense, then from the Reaction, how little soever the duration of it be, a Phantasme or Idea hath its being; which by reason the Endeavour is now outwards, doth alwayes appear as something situate without the Organ. So that now I shall give you the whole Definition of Sense, as it is drawn from the explication of the causes thereof, and the order of its generation, thus; SENSE is a Phantasme, made by the Reaction and endeavour outwards in the Organ of Sense, caused by an Endeavour inwards from the Object, remayning for some time more or less.
3 The Subject of Sense, is the Sentient it self, namely, some living Creature; and we speak more correctly, when we say a Living Creature seeth, then when we say the Eye seeth. The Object, is the thing Perceived; and it is more accurately said, that we see the Sun, then that we see the Light. For Light & Colour & Heat & Sound, and other qualities which are commonly called Sensible, are not Objects, but Phantasms in the Sentients. For a Phantasm is the act of Sense, and differs no otherwise from Sense then fieri (that is, Being a doing) differs from Factum esse, (that is, Being done;) which difference, in things that are done in an Instant, is none at all; and a Phantasme is made in an Instant. For in all Motion which proceeds by perpetual propagation, the first part being moved moves the second, the second the third, and so on to the last, and that to any distance how great soever. And in what point of time the first or formost part proceeded to the place of the second, which is thrust on; in the same point of time the last save one proceeded into the place of the last yeilding part; which by reaction, in the same instant, if the reaction be strong enough, makes a Phantasme; and a Phantasme being made, Perception is made together with it,
4 The Organs of Sense, which are in the Sentient, are such parts thereof, that if they be hurt, the very generation of Phantasmes is thereby destroyed, though all the rest of the parts remain intire. Now these parts in the most of Living Creatures are found to be certain Spirits and Membranes, which proceeding from the Pia Mater, involve the Brain and all the Nerves; also the Brain it self, and the Arteries which are in the Brain; and such other parts, as being stirred, the Hart also, which is the fountain of all Sense is stirred together with them. For whensoever the action of the Object reacheth the Body of the Sentient, that action is by some Nerve propagated to the Brain; and if the Nerve leading thither be so hurt or obstructed, that the Motion can be propagated no further, no Sense follows. Also if the motion be intercepted between the Brain and the Heart by the defect of the Organ by which the action is propagated, there will be no perception of the Object.
5 But though all Sense, as I have said, be made by Reaction, nevertheless it is not necessary that every thing that Reacteth should have Sense. I know there have been Philosophers, & those learned men, who have maintained that all Bodies are endued with Sense. Nor do I see how they can be refuted, if the nature of Sense be placed in Reaction onely. And, though by the Reaction of Bodies inanimate a Phantasme might be made, it would nevertheless cease, as soon as ever the Object were removed. For unless those Bodies had Organs, (as living Creatures have) fit for the retaining of such Motion as is made in them, their Sense would be such, as that they should never reme¯ber the same. And therefore this hath nothing to do with that Sense which is the subject of my discourse. For by Sense we commonly understand the judgement we make of Objects by their Phantasmes; namely, by comparing and distinguishing those Phantasmes; which we could never do, if that motion in the Organ, by which the Phantasme is made, did not remain there for some time, and make the same Phantasme return. Wherefore Sense, as I here understand it, and which is commonly so called, hath necessarily some memory adhering to it, by which former and later Phantasmes may be compared together, and distinguished from one another.
Sense therefore properly so called, must necessarily have in it a perpetual variety of Phantasmes, that they may be discerned one from another. For if we should suppose a man to be made with cleer Eyes, and all the rest of his Organs of Sight well disposed, but endued with no other Sense; and that he should look onely upon one thing, which is alwayes of the same colour and figure without the least appearance of variety, he would seem to me, whatsoever others may say, to see, no more then I seem to my self to feel the Bones of my own Limbs by my Organs of Feeling; and yet those Bones are alwayes, and on all sides touched by a most sensible Membrane. I might perhaps say he were astonished, and looked upon it; but I should not say he saw it; it being almost all one for a man to be alwayes sensible of one and the same thing, and not to be sensible at all of any thing.
6 And yet such is the nature of Sense, that it does not permit a man to discern many things at once. For seeing the nature of Sense consists in Motion; as long as the Organs are employed about one Object, they cannot be so Moved by another at the same time, as to make by both their Motions one sincere Phantasme of each of them at once. And therefore two several Phantasmes will not be made by two Objects working together, but onely one Phantasme compounded from the action of both.
Besides, as when we divide a Body, we divide its place; and when we reckon many Bodies, we must necessarily reckon as many places; and contrarily, as I have shewn in the first Article of the 7th Chapter; so what number soever we say there be of Times, we must understand the same number of Motions also; and as oft as we count many Motions, so oft we reckon many times. For though the object we looke upon be of diverse colours, yet with those diverse colours it is but one varied Object, and not variety of Objects.
Moreover, whilest those Organs which are common to all the Senses (such as are those parts of every Organ which proceed (in Men) from the root of the Nerves, to the Hart) are vehemently stirred by a strong action from some one Object, they are (by reaof the contumacy which the motion they have already gives them against the reception of all other motion) made the lesse fit to receive any other impression from whatsoever other Objects, to what sense soever those Objects belong. And hence it is, that an earnest studying of one Object, takes away the Sense of all other Objects for the present. For Study is nothing else but a possession of the mind, that is to say, a vehement motion made by some one Object in the Organs of Sense, which are stupid to all other motions as long as this lasteth; according to what was said by Terence, Populus studio stupidus in funambulo animum occuparat. For what is Stupor but that which the Greekes call, that is, a cessation from the Sense of other things? Wherefore at one and the same time, we cannot by Sense perceive more then one single Object; as in reading, we see the letters successively one by one, and not all together, though the whole Page be presented to our eye; and though every severall letter be distinctly written there, yet when we looke upon the whole page at once, we read nothing.
From hence it is manifest, that every endeavour of the Organ utwards, is not to be called Sense, but that onely which at severall times is by Vehemence made stronger and more praedominant than the rest; which deprives us of the Sense of other Phantasmes, no otherwise then the Sun deprives the rest of the starres of light, not by hindering their action, but by obscuring and hiding them with his excesse of brightnesse.
- But the motion of the Organ, by which a Phantasme is made, is not commonly called Sense, except the Object be present. And the Phantasme remaining after the Object is removed or past by, is called Fancy, and in latine Imaginatio; which word (because all Phantasmes are not Images) doth not fully answer the signification of the word Fancy in its generall acceptation. Neverthelesse I may use it safely enough, by understanding it for the Greek.
IMAGINATION therefore is nothing else but Sense decaying, or weakned, by the absence of the Object. But what may be the cause of this decay or weakning? Is the Motion the weaker because the Object is taken away? If it were, then Phantasmes would alwayes and necessarily be less cleare in the Imagination, then they are in Sense; which is not true. For in Dreams (which are the Imaginations of those that sleep) they are no less clear then in Sense it self. But the reason why in men Waking the Phantasms of things past are more obscure then those of things present, is this, that their Organs being at the same time moved by other present Objects, those Phantasmes are the lesse praedominant. Whereas in Sleep, the passages being shut up, externall action doth not at all disturbe or hinder internall motion.
If this be true, the next thing to be considered, will be, whether any cause may be found out, from the supposition whereof it will follow, that the passage is shut up from the externall Objects of Sense to the internall Organ. I suppose therefore, that by the continuall action of Objects, (to which a Reaction of the Organ, and more esqecially of the Spirits, is necessarily consequent) the Organ is wearied, that is, its parts are no longer moved by the Spirits without some pain; and consequently the Nerves being abandoned and grown slack, they retire to their fountain which is the cavity either of the Brain or of the Heart; by which means the action which proceeded by the Nerves is necessarily intercepted For Action, upon a Patient that retires from it, makes but little Impression at the first; and at last, when the Nerves are by little and little slacked, none at all. And therefore there is no more Reaction, that is, no more Sense, till the Organ being refreshed by Rest, and by a supply of new Spirits recovering strength and motion, the Sentient awaketh. And thus it seems to be alwayes, unless some other praeternatural cause intervene; as Heat in the internal parts from lassitude, or from some disease stirring the Spirits and other parts of the Organ in some extraordinary manner.
8 Now it is not without cause, nor so casual a thing as many perhaps think it, that Phantasmes in this their great variety, proceed from one another; and that the same Phantasmes sometimes bring into the mind other Phantasmes like themselves, and at other times extreamly unlike. For in the motion of any continued Body, one part followes another by cohaesion; and therefore, whilst we turne our Eies and other Organs successively to many Objects, the motion which was made by every one of them remayning, the Phantasmes are renewed as often as any of those motions comes to be praedominant above the rest; and they become praedominant in the same order, in which at any time formerly they were generated by Sense. So that when by length of time very many Phantasmes have been generated within us by Sense, then allmost any thought may arise from any other thought; in so much that it may seeme to be a thing indifferent and casuall, which thought shall follow which. But for the most part this is not so uncertain a thing to waking as to sleeping men. For the thought or Phantasme of the desired End, brings in all the Phantasmes that are meanes conducing to that end, and that in order backewards from the last to the first, and againe forwards from the beginning to the End: But this supposes both Appetite, and Judgement to discerne what meanes conduce to the end; which is gotten by Experience; and Experience is store of Phantasmes, arising from the sense of very many things. For and Meminisse, Fancy and Memory differ onely in this, that Memory supposeth the time past, which Fancy doth not. In Memory, the Phantasmes we consider are as if they were worne out with time; but in our Fancy we consider them as they are; which distinction is not of the things themselves, but of the considerations of the Sentient. For there is in Memory, something like that which happens in looking upon things at a great distance; in which, as the small parts of the Object are not discerned by reason of their remotenesse; so in Memory, many accidents and places and parts of things, which were formerly perceived by Sense, are by length of time decayed and lost.
The perpetuall arising of Phantasmes, both in Sense and Imagination, is that which we commonly call Discourse of the Mind, and is common to men with other living Creatures. For he thta thinketh, compareth the Phantasmes that passe, that is, taketh notice of their likenesse or unlikenesse to one another. And as he that observes readily the likenesses of things of different natures, or that are very remote from one another, is said to have a good Fancy; so he is said to have a good Judgement, that finds out the unlikenesses or differences of things that are like one another. Now this observation of differences, is not perception made by a common Organ of Sense, distinct from Sense or Perception properly so called; but is Memory of the differences of particular Phantasmes remaining for some time; as the distinction between Hot and Lucid, is nothing else but the Memory both of a Heating, and of an Enlightning Object.
9 The Phantasmes of men that sleep, are DREAMS. Concerning which we are taught by experience these five things. First, that for the most part there is neither order nor coherence in them. Secondly, that we dream of nothing but what is compounded and made up of the Phantasmes of Sense past. Thirdly, that somtimes they proceed (as in those that are drowsy) from the interruption of their Phantasmes by little and little broken and altered through sleepiness; and sometimes also they begin in the midst of sleep. Fourthly, that they are clearer then the Imaginations of waking men, except such as are made by Sense itself, to which they are equal in clearness. Fifthly, that when we dream, we admire neither the places nor the looks of the things that appear to us. Now from what hath been said, it is not hard to shew what may be the causes of these Phaenomena. For as for the first, seeing all Order and Coherence proceeds from frequent looking back to the End, that is, from Consultation; it must needs be, that seeing in sleep we lose all thought of the End, our Phantasmes succeed one another, not in that order which tends to any End, but as it hapneth, and in such manner, as Objects present themselves to our Eyes when we look indifferently upon all things before us, and see them, not because we would see them, but because we do not shut our Eyes; for then they appear to us without any order at all. The second proceeds from this, that in the silence of Sense, there is no new motion from the Objects, and therefore no new Phantasme, unless we call that new, which is compounded of old ones, as a Chimaera, a golden Mountain, and the like. As for the third, why a Dream is sometimes as it were the continuation of Sense, made up of broken Phantasmes, as in men distempered with sickness, the reason is manifestly this, that in some of the Organs Sense remains, and in others it faileth. But how some Phantasmes may be revived, when all the exteriour Organs are benummed with sleep, is not so easily shewn. Nevertheless, that which hath already been said, contains the reason of this also. For whatsoever strikes the Pia Mater, reviveth some of those Phantasmes that are still in motion in the Brain; and when any internal motion of the Heart reacheth that Membrane, then the praedominant motion in the Brain makes the Phantasme. Now the Motions of the Heart are Appetites and Aversions, of which I shall presently speak further. And as Appetites and Aversions are generated by Phantasmes, so reciprocally Phantasmes are generated by Appetites and Aversions. For example, Heat in the Heart proceeds from Anger and Fighting; and again from Heat in the Heart, (whatsoever be the cause of it) is generated Anger, and the Image of an Enemy in Sleep. And as Love and Beauty stirre up heat in certain Organs; so Heat in the same Organs, from whatsoever it proceeds, often causeth Desire, and the Image of an unresisting Beauty. Lastly, Cold doth in the same manner generate Feare in those that sleep, and causeth them to dream of Ghosts, and to have Phantasmes of horrour and danger; as Fear also causeth Cold in those that wake; so reciprocal are the motions of the Heart and Brain. The fourth, namely, that the things we seem to see and feel in sleep, are as clear as in sense it self, proceeds from two causes; one, that having then no sense of things without us, that internal motion which makes the Phantasme, in the absence of all other impressions, is praedominant; and the other, that the parts of our Phantasms which are decayed and worn out by time, are made up with other fictitious parts. To conclude, when we dream, we do not wonder at strange places, and the appearances of things unknown to us, because Admiration requires that the things appearing be new and unusual, which can happen to none but those that remember former appearances; whereas in sleep, all things appear as present.
But it is here to be observed, that certain Dreams, especially such as some men have when they are between sleeping and waking, and such as happen to those that have no knowledge of the nature of Dreams, and are with all superstitious, were not heretofore, nor are now accounted Dreams. For the Apparitions men thought they saw, and the Voices they thought they heard in sleep, were not believed to be Phantasmes, but things subsisting of themselves, and Objects without those that dreamed. For to some men, as well sleeping as waking, but especially to guilty men, and in the night, and in hallowed places, Feare alone, helped a little with the stories of such Apparitions, hath raised in their minds terrible Phantasmes, which have been, and are still deceiptfully received for things really true, under the names of Ghosts and Incorporeal Substances.
10 In most living Creatures there are observed five kinds of Senses, which are distinguished by their Organs, and by their different kinds of Phantasmes; namely, Sight, Hearing, Smell, Tast and Touch; and these have their Organs partly peculiar to each of them severally, and partly common to them all. The Organ of Sight is partly animate, and partly inanimate. The inanimate parts, are the three Humours; namely, the Watry Humour, which by the interposition of the Membrane called Uea, (the perforation whereof is called the Apple of the Eye) is contained on one side by the first concave superficies of the Eye, and on the other side by the Ciliary processes and the Coat of the Cristalline humour; the Cristalline, which (hanging in the midst between the Ciliary processes, and being almost of Spherical figure, and of a thick consistence) is inclosed on all sides with its own transparent Coat; and the Vitreous or Glassie Humour, which filleth all the rest of the Cavity of the Eye, and is somewhat thicker then the Watry Humour, but thinner then the Cristalline. The animate part of the Organ is, first, the Membrane Choroeides, which is a part of the Pia Mater, saving that it is covered with a Coat derived from the marrow of the Optique Nerve, which is called the Retina; and this Choroeides, seeing it is part of the Pia Mater, is continued to the beginning of the Medulla Spinalis within the Scull, in which all the Nerves which are within the Head have their roots. Wherefore all the Animal Spirits that the Nerves receive, enter into them there, for it is not imaginable that they can enter into them any where else. Seeing therefore Sense is nothing else but the action of Objects propagated to the furthest part of the Organ; and seeing also that Animal Spirits are nothing but Vital Spirits purified by the Hart and carried from it by the Arteries; it follows necessarily, that the action is derived from the Heart by some of the Arteries to the roots of the Nerves which are in the Head, whether those Arteries be the Plexus Retiformis, or whether they be other Arteries which are inserted into the substance of the Brain. And therefore those Arteries are the Complement, or the remaying part of the whole Organ of Sight. And this last part is a common Organ to all the Senses; wheras that which reacheth from the Eie to the roots of the Nerves is proper onely to Sight. The proper Organ of Hearing is the Tympanum of the Eare, and its own Nerve; from which to the Heart the Organ is Common. So the proper Organs of Smel & Tast are Nervous Membranes, in the Palate and Tongue for the Taste, and in the Nostrils for the Smell; and from the roots of those Nerves to the Heart all is common. Lastly, he proper Organ of Touch are Nerves and Membranes dispersed through the whole Body; which Membranes are derived from the root of the Nerves. And all things else belonging alike to all the Senses seem to be administred by the Arteries, and not by the Nerves.
The proper Phantasme of Sight is Light; and under this name of Light, Colour also (which is nothing but perturbed Light) is comprehended. Wherefore the Phantasme of a Lucid Body, is Light; and of a coloured Body, Colour. But the Object of Sight, properly so called is neither Light nor Colour, but the Body itself which is lucid, or enlightned, or coloured. For Light and Colour being Phantasmes of the Sentient, cannot be Accidents of the Object. Which is manifest enough from this, that Visible things appear oftentimes in places, in which we know assuredly they are not, and that in different places they are of different colours, and may at one and the same time appear in divers places. Motion, Rest, Magnitude and Figure are common both to the Sight and Touch; and the whole appearance together of Figure, and Light or Colour, is by the Greeks commonly called and by the Latines, Species and Imago; all which names signifie no more but Appearance.
The phantasme which is made by Hearing, is Sound; by Smell, Odour; by Tast, Savour; and by Touch, Hardness and Softness, Heat and Cold, Wetness, Oiliness, and many more, which are easier to be distinguished by sense then words. Smoothness, Roughness, Rarity and Density refer to Figure, and are therefore common both to Touch and Sight. And as for the Objects of Hearing, Smel, Tast, and Touch, they are not Sound, Odour, Savour, Hardness, &c. but the Bodies themselves from which Sound, Odour, Savour, Hardness, &c. proceed; Of the causes of which, and of the manner how they are produced, I shall speak hereafter.
But these Phantasmes, though they be effects in the Sentient, (as Subject) produced by Objects working upon the Organs; yet there are also other effects besides these, produced by the same Objects, in the same Organs; namely, certain Motions proceeding from Sense, which are called Animal Motions. For seeing in all Sense of external things, there is mutual Action and Reaction, that is, two Endeavours opposing one another, it is manifest, that the motion of both of them together will be continued every way, especially to the confines of both the Bodies. And when this happens in the internal Organ, the Endeavour outwards will proceed in a solid Angle, which will be greater, and consequently the Idea greater, then it would have been if the impression had been weaker.
11 From hence the Natural cause is manifest, First, why those things seem to be greater, which (caeteris paribus) are seen in a greater Angle. Secondly, why in a serene cold night when the Moon doth not shine, more of the fixed Stars appear, then at another time. For their action is less hindred by the serenity of the Aire, and not obscured by the greater Light of the Moon, which is then absent; and the Cold making the Air more pressing, helpeth or strengtheneth the action of the Stars upon our Eies, in so much as Stars may then be seen which are seen at no other time. And this may suffice to be said in general concerning Sense made by the Reaction of the Organ. For, as for the place of the Image, the deceptions of Sight, and other things of which we have experience in our selves by Sense, being they depend for the most part upon the Fabrick it self of the Eie of Man, I shall speak of them then when I come to speak of Man.
12 But, there is another kind of Sense, of which I will say somthing in this place, namely, the Sense of Pleasure and Pain, proceeding, not from the Reaction of the Heart outwards, but from continual action from the outermost part of the Organ towards the Heart. For the original of Life being in the Heart, that motion in the Sentient which is propagated to the Heart, must necessarily make some alteration or diversion of Vital Motion, namely, by quickning or slackening, helping or hindering the same. Now when it helpeth, it is Pleasure, and when it hindereth, it is Pain, Trouble, Grief, &c. And as Phantasmes seem to be without, by reason of the Endeavour outwards; so Pleasure and Pain by reason of the Endeavour of the Organ inwards seem to be within, namely, there where the first Cause of the Pleasure or Pain is; as when the Pain proceeds from a Wound, we think the Pain and the Wound are both in the same place.
Now Vital Motion, is the Motion of the Bloud, perpetually circulating (as hath been shewn from many infallible signes and marks by Doctor Harvey, the first Observer of it) in the Veins and Arteries. Which Motion, when it is hindered by some other Motion made by the action of sensible Objects, may be restored again either by bending or setting straight the parts of the Body; which is done when the Spirits are carried now into these, now into other Nerves, till the Pain (as farre as is possible) be quite taken away. But if Vital Motion be helped by Motion made by Sense, then the parts of the Organ will be disposed to guide the Spirits in such manner, as conduceth most to the preservation and augmentation of that motion by the help of the Nerves. And in animal motion this is the very first Endeavour, and found even in the Embrio; which while it is in the wombe, moveth its limbes with voluntary motion, for the avoiding of whatsoever troubleth it, or for the pursuing of what pleaseth it. And this first Endeavour, when it tends towards such things as are known by experience to be pleasant, is called APPETITE, (that is, an Approaching;) and when it shuns what is troublesome, AVERSION, or Flying from it. And little Infants, at the beginning, and as soon as they are born, have appetite to very few things, as also they avoid very few, by reason of their want of Experience and Memory; & therefore they have not so great a variety of animal Motion as we see in those that are more grown. For it is not possible, without such knowledge as is derived from Sense, that is, without Experience and Memory, to know what will prove pleasant, or hurtful; onely there is some place for conjecture from the looks or aspects of things. And hence it is, that though they do not know what may do them good or harm, yet sometimes they approach, and sometimes retire from the same thing as their doubt prompts them. But afterwards by accustoming themselves by little and little, they come to know readily what is to be pursued, and what to be avoided; and also to have a ready use of their Nerves and other Organs in the pursuing and avoiding of good and bad. Wherefore Appetite and Aversion are the first Endeavours of Animal Motion.
Consequent to this first Endeavour, is the Impulsion into the Nerves, and Retraction again of Animal Spirits, of which it is necessary there be some Receptacle on place neer the original of the Nerves; and this Motion or Endeavour is followed by a swelling and Relaxation of the Muscles; and lastly, these are followed by Contraction and Extension of the limbes, which is Animal Motion.
13 The Considerations of Appetites and Aversions are divers. For seeing Living Creatures have sometimes Appetite, and sometimes Aversion to the same thing, as they think it will either be for their good or their hurt, while that vicissitude of Appetites and Aversions remains in them they have that series of Thoughts which is called DELIBERATION; which lasteth as long as they have it in their power to obtain that which pleaseth, or to avoid that which displeaseth them. Appetite therefore and Aversion are simply so called as long as they follow not Deliberation. But if Deliberation have gone before, then the last act of it, if it be Appetite, is called WILL; if Aversion, UNWILLINGNESSE; so that the same thing is called both Will and Appetite; but the consideration of them (namely, before and after Deliberation) is divers. Nor is that which is done within a Man whilest he Willeth any thing, different from that which is done in other living Creatures, whilest (Deliberation having preceded) they have Appetite.
Neither is the freedome of Willing, or not willing, greater in Man, then in other living Creatures. For where there is Appetite, the entire cause of Appetite hath preceded; and consequently the act of Appetite could not choose but follow, that is, hath of necessity followed, (as is shewn Chapt. 9th Article 5.) And therefore such a Liberty as is free from Necessity, is not to be found in the Will either of Men or Beasts. But if by Liberty we understand the faculty or power, not of Willing, but of Doing what they Will, then certainly that Liberty is to be allowed to both; and both may equally have it, whensover it is to be had.
Again, when Appetite and Aversion do with celerity succeed one another, the whole series made by them, hath its name sometimes from one, sometimes from the other. For the same Deliberation (whilest it inclines sometimes to one, sometimes to the other) is from Appetite called HOPE, and from Aversion FEAR. For where there is no Hope, it is not to be called Fear, but HATE; and where no Fear, not Hope, but DESIRE. To conclude, all the Passions, called Passions of the Minde, consist of Appetite and Aversion (except pure Pleasure and Pain, which are a certain Fruition of good or Evil;) as Anger, is Aversion from some imminent evil, but such as is joyned with Appetite of avoiding that evil by force. But because the Passions and Perturbations of the Minde are innumerable; and many of them not to be discerned in any Creatures besides Men; I will speak of them more at large in that Section which is concerning Man. As for those Objects (if there be any such) which do not at all stir the Mind, we are said to Contemn them.
And thus much of Sense in general. In the next place I shall speak of Sensible Objects.
CHAP. XXVI. OF THE WORLD, AND OF THE STARRES.
1 THE MAGNITUDE and Duration of the World, inscrutable.
2 No place in the World Empty.
3 The arguments of Lucretius for Vacuum, invalid.
4 Other arguments for the establishing of Vacuum, invalid.
5 Six suppositions for the salving of the Phaenomena of Nature.
6 Possible causes of the Motions Annual and Diurnal; and of the apparent Direction, Station and Retrogradation of the Planets.
7 The supposition of Simple Motion, why likely.
8 The cause of the Excentricity of the annual motion of the Earth.
9 The cause why the Moon hath alwayes one and the same face turned towards the Earth.
10 The cause of the Tides of the Ocean.
11 The cause of the Praecession of the Equinoxes.
1 CONSEQUENT to the Contemplation of Sense is the contemplation of Bodies, which are the efficient causes or Objects of Sense. Now every Object is either a part of the whole World, or an Aggregate of parts. The greatest of all Bodies, or sensible Objects, is the World it self, which we behold when we look round about us from this point of the same which we call the Earth. Concerning the World, as it is one Aggregate of many parts, the things that fall under inquiry are but few; and those we can determine, none. Of the whole World we may inquire what is its Magnitude, what its Duration, and how many there be; but nothing else. For as for Place and Time, that is to say, Magnitude and Duration, they are only our own fancies of a Body simply so called, that is to say of a Body indefinitely taken, as I have shewne before in the 7 chapter. All other Phantasmes, are of Bodies or Objects as they are distinguished from one another; as Colour, the Phantasme of coloured Bodies; Sound, of Bodies that move the Sense of Hearing, &c. The questions concerning the Magnitude of the World, are, whether it be Finite or Infinite, Full or not Full; Concerning its Duration, whether it had a Beginning, or be Eternall; and concerning the number, whether there be One or Many (though as concerning the Number, if it were of infinite Magnitude, there could be no controversy at all.) Also if it had a beginning, then by what Cause and of what Matter it was made; and againe, from whence that Cause and that Matter had their being, will be new questions; till at last we come to one or many eternall Cause or Causes. And the determination of all these things belongeth to him that professeth the universal doctrine of Philosophy, in case as much could be known as can be sought. But the knowledge of what is Infinite can never be attained by a finite Inquirer. Whatsoever we know that are Men, we learn it from our Phantasmes; and of Infinite (whether Magnitude or Time) there is no Phantasme at all; so that it is impossible either for a man, or any other creature to have any conception of Infinite. And though a man may from some Effect proceed to the immediate Cause thereof, & fro¯ that to a more remote Cause, and so ascend continually by right ratiocination from Cause to Cause; yet he will not be able to proceed eternally; but wearied, will at last give over, without knowing whether it were possible for him to proceed to an end, or not. But whether we suppose the World to be Finite, or Infinite, no absurdity will follow. For the same things which now appear, might appear, whether the Creator had pleased it should be Finite or Infinite. Besides, though from this, that nothing can move it self, it may rightly be inferred that there was some first eternal Movent; yet it can never be inferred (though some use to make such inference) that that Movent was eternally Immoveable, but rather eternally Moved. For as it is true, that nothing is moved by it self; so it is true also that nothing is moved but by that which is already moved. The questions therefore about the Magnitude and Beginning of the World, are not to be determined by Philosophers, but by those that are lawfully authorised to order the Worship of God. For as Almighty God, when he had brought his People into Judaea, allowed the Priests the first fruits reserved to himself; so when he had delivered up the World to the disputations of Men, it was his pleasure, that all Opinions concerning the nature of Infinite and Eternal, known onely to himself, should (as the first-fruits of Wisdom) be judged by those whose Ministery he meant to use in the ordering of Religion. I cannot therefore commend those that boast they have demonstrated by reasons drawn from natural things, that the World had a Beginning. They are contemned by Idiots, because they understand them not; and by the Learned, because they understand them; by both deservedly. For who can commend him that demonstrates thus? If the World be Eternal, then an infinite number of dayes (or other measures of Time) preceded the birth of Abraham. But the birth of Abraham preceded the birth of Isaac; and therefore one Infinite is greater then another Infinite, or one Eternal then another Eternal; which (he sayes) is absurd. This Demonstration is like his, who from this, that the number of even Numbers is infinite, would conclude, that there are as many even Numbers, as there are Numbers simply, that is to say, the even Numbers are as many, as all the even and od together. They which in this manner take away Eternity from the World, do they not by the same means take away Eternity from the Creator of the Wold? From this absurdity therefore they run into another, being forced to call Eternity Nunc stans, a standing still of the present Time, or an abiding Now; and (which is much more absurd) to give to the infinite number of Numbers, the name of Unity. But why should Eternity be called an Abiding Now, rather then an Abiding Then? Wherefore there must either be many Eternities, or Now and Then must signifie the same. With such Demonstrators as these, that speak in another language, it is impossible to enter into disputation. And the men that reason thus absurdly, are not Idiots, but (which makes the absurdity unpardonable) Geometricians, and such as take upon them to be Judges (impertinent, but severe Judges) of other mens Demonstrations. The reason is this, that as soon as they are entangled in the Words Infinite and Eternal, (of which we have in our mind no Idea, but that of our own insufficiency to comprehend them) they are forced either to speak something absurd, or (which they love worse) to hold their peace. For Geometry hath in it somewhat like Wine; which when new, is windy; but afterwards though less pleasant, yet more wholsome. Whatsoever therefore is true, young Geometricians think Demonstrable; but elder not. Wherefore I purposely pass over the Questions of Infinite and Eternal; contenting my self with that Doctrine concerning the Beginning and Magnitude of the World, which I have been perswaded to by the holy Scriptures, and fame of the Miracles which confirm them; and by the Custome of my Country, and reverence due to the Laws. And so I pass on to such things as it is not unlawful to dispute of.
2 Concerning the World it is further questioned, whether the parts thereof be contiguous to one another, in such manner, as not to admit of the least empty space between; and the disputation both for & against it, is carried on with probability enough. For the taking away of Vacuum, I will instance in onely one experiment, a common one, but (I think) unanswerable.
Let AB (in the first fig.) represent a vessel (such as Gardiners use to water their Gardens withal;) whose bottom B is ful of litle holes; & whose mouth A may be stopt with ones finger when there shall be need. If now this vessel be filled with water, the hole at the top A being stopt, the water will not flow out at any of the holes in the bottom B. But if the finger be removed to let in the air above, it will run out at them all; and as soon as the finger is applyed to it again, the water wil suddenly & totally be stayed again from running out. The cause whereof seems to be no other but this, that the Water cannot by its natural endeavour to descend, drive down the aire below it, because there is no place for it to go into, unless either by thrusting away the next contiguous aire it proceed by continual endeavour to the hole A, where it may enter and succeed into the place of the water that floweth out; or else by resisting the endeavour of the water Downwards, penetrate the same, and pass up through it. By the first of these wayes (while the hole at A remains stopped) there is no possible passage; nor by the second, unless the holes be so great, that the water flowing out at them, can by its own waight force the Air at the same time to ascend into the vessel by the same holes; as we see it does in a vessel whose mouth is wide enough, when we turn suddenly the bottom upwards to poure out the water; for then the Aire being forced by the waight of the water, enters (as is evident by the sobbing and resistance of the water) at the sides or circumference of the orifice. And this I take for a sign that all Space is full; for without this the naturall motion of the water (which is a heavy Body) downwards, would not be hindered.
3 On the contrary, for the establishing of Vacuum, many & specious arguments and experiments have been brought. Neverthelesse there seemes to be something wanting in all of them to conclude it firmely. These arguments for Vacuum are partly made by the followers of the doctrine of Epicurus; who taught that the World consists of very small Spaces not filled by any Bodie, and of very small Bodies that have within them no empty Space, (which by reason of their hardnesse he calls Atomes); and that these small Bodies and Spaces are every where intermingled. Their arguments are thus delivered by Lucretius.
And first he sayes that unlesse it were so, there could be no motion; For the office and property of Bodies is to withstand and hinder motion. If therfore the Universe were filled with Body, motion would every where be hindered, so, as to have no beginning any where; & consequently there would be no motion at all. It is true that in whatsoever is full, and at rest in all its parts, it is not possible motion should have beginning. But nothing is drawn from hence for the proving of Vacuum. For though it should be granted that there is Vacuum; yet if the Bodies which are intermingled with it, should all at once and together be at rest, they would never be moved again. For it has been demonstrated above (in the 9th Chapter 7th Article) that nothing can be moved but by that which is contiguous and already moved. But supposing that all things are at rest together, there can be nothing contiguous and moved; and therefore no beginning of motion. Now the denying of the beginning of motion, doth not take away present motion, unless beginning be taken away from Body also. For motion may be either coeternal, or concreated with Body. Nor doth it seem more necessary that Bodies were first at rest, and afterwards moved, then that they were first moved, and rested (if ever they rested at all) afterwards. Neither doth there appear any cause, why the matter of the World should for the admission of motion, be intermingled with empty spaces, rather then full; I say full, but withall fluid. Nor lastly, is there any reason why those hard Atomes may not also by the motion of intermingled fluid matter be congregated & brought together into compounded Bodies of such bigness as we see. Wherefore nothing can by this argument be concluded, but that motion was either coeternal, or of the same duration with that which is moved; neither of which conclusions consisteth with the doctrine of Epicurus, who allows neither to the World nor to Motion any Beginning at all. The necessity therefore of Vacuum is not hitherto demonstrated. And the cause (as far as I understand from them that have discoursed with me of Vacuum) is this, that whilest they contemplate the nature of Fluid, they conceive it to consist as it were of small grains of hard matter, in such manner as meal is fluid, made so by grinding of the Corn; when nevertheless it is possible to conceive Fluid to be of its own nature as homogeneous, as either an Atome, or as Vacuum, it self.
The second of their arguments is taken from waight, and is contained in these Verses of Lucretius,
Corporis officium est quoniam premere omnia deorsum;
Contrà autem natura manet sine Pondere Inanis;
Ergo quod magnum est aeque, Leviusque videtur,
Nimirum plus esse sibi declarat Inanis.
That is to say, Seeing the office and property of Body is to press all things downwards; and on the contrary, seeing the nature of Vacuum is to have no waight at all; Therefore when of two Bodies of equal magnitude, one is lighter then the other, it is manifest that the lighter Body hath in it more Vacuum then the other.
To say nothing of the Assumption concerning the endeavour of Bodies downwards, which is not rightly assumed, because the World hath nothing to do with Downwards, which is a mere fiction of ours; Nor of this, that if al things tended to the same lowest part of the World, either there would be no coalescence at all of Bodies, or they would all be gathered together into the same place. This onely is sufficient to take away the force of the argument, that Aire intermingled with those his Atomes, had served as well for his purpose, as his intermingled Vacuum.
The third argument is drawn from this, That Lightning, Sound, Heat and Cold do penetrate all Bodies (except Atomes) how solid soever they be. But this reason, except it be first demonstrated that the same things cannot happen (without Vacuum) by perpetual generation of Motion, is altogether invalid. But that all the same things may so happen, shall in due place be demonstrated.
Lastly, the fourth argument is set down by the same Lucretius in these Verses.
Duo de concursu corpora lata
Si citò dissiliant, nempe aer omne necesse est
Inter corpora quod fuerat, possidat Inane.
Is porro quamvis circum celerantibus auris
Confluat, haud poterit tamen uno tempore totum
Compleri spatium; nam primum quemque necesse est
Occupet ille locum, deinde omnia possideantur.
That is, If two flat Bodies be suddenly pulled asunder, of necessity the Air must come between them to fill all the space they left empty. But with what celerity soever the Air flow in, yet it cannot in one instant of time fill the whole space, but first one part of it, then successively all.
Which nevertheless is more repugnant to the opinion of Epicurus, then of those that deny Vacuum. For though it be true, that if two Bodies were of infinite hardness, and were joyned together by their Superficies which were most exactly plain, it would be impossible to pull them asunder, in regard it could not be done but by Motion in an instant; yet, if as the greatest of all Magnitudes cannot be given, nor the swiftest of all Motions, so neither the hardest of all hard Bodies; it might be, that by the application of very great force, there might be place made for a successive flowing in of the Aire, namely by separating the parts of the joyned Bodies by succession, beginning at the outermost and ending at the innermost part. He ought therefore first to have proved, that there are some Bodies extreamly hard, not relatively, as compared with softer Bodies, but absolutely, that is to say, infinitely hard; which is not true. But if we suppose (as Epicurus doth) that Atomes are indivisible, and yet have small superficies of their own; then if two Bodies should be joyned together by many, or but one onely small superficies of either of them, then I say this argument of Lucretius would be a firme demonstration, that no two Bodies made up of Atomes (as he supposes) could ever possibly be pulled asunder by any force whatsoever. But this is repugnant to daily experience.
And thus much of the arguments of Lucretius. Let us now consider the arguments which are drawn from the experiments of later Writers.
4 The first experiment is this, That if a hollow vessel be thrust into water with the bottom upwards, the water will ascend into it; which they say it could not do, unless the Aire within were thrust together into a narrower place; and that this were also impossible except there were little empty places in the Aire. Also that when the Aire is compressed to a certain degree, it can receive no further compression, its small particles not suffering themselves to be pent into less room. This reason, if the Aire could not pass through the Water as it ascends within the vessel, might seem valid. But it is sufficiently known, that Aire will penetrate Water by the application of a force equal to the gravity of the Water. If therefore the force by which the Vessel is thrust down, be greater, or equal to the endeavour by which the water naturally tendeth downwards, the Aire will go out that way where the resistance is made, namely, towards the edges of the Vessel. For, by how much the deeper is the water which is to be penetrated, so much greater must be the depressing force. But after the Vessel is quite under water, the force by which it is depressed, that is to say, the force by which the water riseth up, is no longer encreased. There is therefore such an equilibration between them, as that the natural endeavour of the water downwards, is equal to the endeavour by which the same water is to be penetrated to the encreased depth.
The second experiment is, That if a concave Cylinder of sufficient length (made of Glass, that the experiment may be the better seen) having one end open, and the other close shut, be filled with Quicksilver, and the open end being stopped with ones finger, be together with the finger dipped into a dish or other vessel in which also there is Quicksilver, and the Cylinder be set upright, we shall, the finger being taken away (to make way for the descent of the Quicksilver) see it descend into the Vessel under it, till there be onely so much remayning within the Cylinder as may fill about 26 Inches of the same; and thus it will alwayes happen whatsoever be the Cylinder, provided that the length be not less then 26 Inches. From whence they conclude that the cavity of the Cylinder above the Quicksilver remayns empty of all Body. But in this experiment I finde no necessity at all of Vacuum. For when the Quicksilver which is in the Cylinder descends, the Vessel under it must needs be filled to a greater height, and consequently so much of the contiguous Air must be thrust away as may make place for the Quicksilver which is descended. Now if it be asked whether that Aire goes, what can be answered but this, that it thrusteth away the next Aire, & that the next, & so successively, till there be a return to the place where the propulsion first began? and there, the last Aire thus thrust on will press the Quicksilver in the Vessel with the same force with which the first Aire was thrust away; and if the force with which the Quicksilver descends be great enough (which is greater or less, as it descends from a place of greater or less height) it will make the Aire penetrate the Quicksilver in the vessel, and go up into the Cylinder to fill the place which they thought was left empty. But because the Quicksilver hath not in every degree of height force enough to cause such penetration, therefore in descending it must of necessity stay somewhere, namely there, where its endeavour downwards, and the resistance of the same to the penetration of the Aire come to an aequilibrium. And by this experiment it is manifest, that this aequilibrium will be at the height of 26 Inches, or thereabouts.
The third experiment is, That when a Vessel hath as much Air in it as it can naturally contain, there may nevertheless be forced into it as much water as will fill three quarters of the same Vessel. And the experiment is made in this manner. Into the glass bottle, represented (in the 2d figure) by the Sphere F G, whose center is A, let the pipe B A C be so fitted, that it may precisely fill the mouth of the bottle; and let the end B, be so neer the bottom, that there may be onely space enough left for the free passage of the water which is thrust in above. Let the upper end of this pipe have a Cover at D, with a spout at E, by which the water (when it ascends in the pipe) may run out. Also let H C be a Cock, for the opening or shutting of the passage of the water between B and D, as there shall be occasion. Let the Cover D E be taken off; and the Cock H C being opened, let a Syringe fall of water be forced in; and before the Syringe be taken away, let the Cock be turned to hinder the going out of the Aire. And in this manner let the injection of water be repeated as often as it shall be requisite, till the water rise within the bottle, for example, to G F. Lastly, the Cover being fastened on again, and the Cock H C opened, the water will run swiftly out at E, and sink by little and little from G F to the bottom of the pipe B.
From this Phaenomenon they argue for the necessity of Vacuum in this manner. The Bottle from the beginning was full of Aire; which Aire could neither go out by penetrating so great a length of water as was injected by the pipe, nor by any other way. Of necessity therefore all the water as high as F G, as also all the Aire that was in the bottle before the water was forced in, must now be in the same place, which at first was filled by the Aire alone; which were impossible, if all the space within the bottle were formerly filled with Aire precisely, that is, without any Vacuum. Besides, though some man perhaps may think the Air, being a thinne Body, may pass through the Body of the water contained in the pipe, yet from that other Phaenomenon, (namely, that all the water which is in the space B F G, is cast out again by the spout at E, for which it seems impossible that any other reason can be given besides the force by which the Aire frees it self from compression) it follows, that either there was in the bottle some space empty, or that many Bodies may be together in the same place. But this last is absurd; and therefore the former is true, namely, that there was Vacuum.
This argument is infirm in two places. For first that is assumed which is not to be granted; and in the second place an experiment is brought, which I think is repugnant to Vacuum. That which is assumed is, that the Aire can have no passage out through the pipe. Nevertheless we see daily that Aire easily ascends from the bottom to the Superficies of a River (as is manifest by the bubbles that rise); nor doth it need any other cause to give it this motion, then the natural endeavour downwards of the Water. Why therefore may not the endeavour upwards of the same Water acquired by the injection (which endeavour upwards is greater then the natural endeavour of the water downwards) cause the aire in the bottle to penetrate in like manner the water that presseth it downwards; especially seeing the water as it riseth in the bottle, doth so press the Aire that is above it, as that it generateth in every part thereof an endeavour towards the external superficies of the pipe, and consequently maketh all the parts of the enclosed aire to tend directly towards the passage at B? I say this is no less manifest, then that he aire which riseth up from the bottom of a River should penetrate the water, how deep soever it be. Werefore I do not yet see any cause why the force by which the water is injected, should not at the same time eject the aire.
And as for their arguing the necessity of Vacuum from the rejection of the water; In the first place, supposing there is Vacuum, I demand by what principle of motion that ejection is made. Certainly, seeing this motion is from within outwards, it must needs be caused by some Agent within the bottle; that is to say, by the aire it self. Now the motion of that aire, being caused by the rising of the water, begins at the bottom, and tends upwards; whereas the motion by which it ejecteth the water ought to begin above, and tend downwards. From whence therefore hath the enclosed aire this endeavour towards the bottom. To this question I know not what answer can be given, unless it be said, that the aire descends of its own accord to expel the water. Which because it is absurd, and that the aire after the water is forced in, hath as much room as its magnitude requires, there will remain no cause at all why the water should be forced out. Wherefore the assertion of Vacuum is repugnant to the very experiment which is here brought to establish it,
Many other Phaenomena are usually brought for Vacuum, as those of Weather-glasses, Aeolipiles, Wind-guns, &c. Which would all be very hard to be salved, unless water be penetrable by aire, without the intermixture of empty space. But now, seeing aire may with no great endeavour pass through, not onely water, but any other fluid Body, though never so stubborn, as Quicksilver, these Phaenomena prove nothing. Nevertheless, it might in reason be expected, that he that would take away Vacuum, should without Vacuum shew us such causes of these Phaenomena, as should be at least of equal, if not greater probability. This therefore shall be done in the following discourse, when I come to speak of these Phaenomena in their proper places. But first the most general Hypotheses of natural Philosophy are to be premised.
And seeing that Suppositions are put for the true Causes of apparent Effects, every Supposition, except such as be absurd, must of necessity consist of some supposed possible Motion (for Rest can never be the Essicient Cause of any thing), & Motion supposeth Bodies Moveable; of which there are three kinds, Fluid, Consistent, and mixt of both. Fluid are those, whose parts may by very weak endeavonr be separated from one another; and Consistent those, for the separation of whose parts greater force is to be applyed. There are therefore degrees of Consistency; which degrees, by comparison with more or less Consistent, have the names of Hardness, or Softness. Wherefore a Fluid Body is alwayes divisible into Bodies equally Fluid, as Quantity into Quantities; and Soft Bodies, of whatsoever degree of Softness, into Soft Bodies of the same degree. And though many men seem to conceive no other difference of Fluidity, but such as ariseth from the different magnitudes of the parts (in which Sense, Dust, though of Diamonds, may be called Fluid); Yet I understand by Fluidity, that which is made such by Nature equally in every part of the Fluid Body; not as Dust is Fluid, for so a House which is falling in pieces may be called Fluid; but in such manner as Water seems Fluid, and to divide it self into parts perpetually Fluid. And this being well understood, I come to my Suppositions.
5 First, therefore I suppose, That the Immense Space which we call the World, is the Aggregate of all Bodies; which are either Consistent & Visible, as the Earth and the Starres; or Invisible, as the small Atomes which are disseminated through the whole space between the Earth and the Stars; and lastly, that most Fluid Aether, which so fils all the rest of the Universe, as that it leaves in it no empty place at all.
Secondly, I suppose with Copernicus, That the greater Bodies of the World, which are both consistent and permanent, have such order amongst themselves, as that the Sunne hath the first place, Mercury the second, Venus the third, The Earth (with the Moon going about it) the fourth, Mars the fifth, Jupiter (with his Attendants) the sixth, Saturne the seventh, and after these the Fixed Starres have their several distances from the Sunne.
Thirdly, I suppose, That in the Sunne & the rest of the Planets, there is and alwayes has been a Simple Circular Motion.
Fourthly, I suppose, That in the Body of the Aire there are certain other Bodies intermingled, which are not Fluid; but withal that they are so small, that they are not preceptible by Sense, and that these also have their proper Simple Motion; and are some of them more, some less hard or consistent.
Fifthly, I suppose with Kepler, That as the distance between the Sunne and the Earth, is to the distance between the Moon and the Earth; so the distance between the Moon and the Earth, is to the Semidiameter of the Earth.
As for the Magnitude of the Circles, and the Times in which they are described by the Bodies which are in them, I will suppose them to be such as shall seem most agreeable to the Phaenomena in question,
6 The causes of the different Seasons of the Year, and of the several variations of Dayes and Nights in all the parts of the superficies of the Earth, have been demonstrated first by Copernicus, and since by Kepler, Galilaeus and others, from the supposition of the Earths diurnal revolution about its own Axis, together with its Annual motion about the Sunne, in the Ecliptick according to the order of the Signes; and thirdly, by the annual revolution of the same Earth about its own center contrary to the order of the Signs. I suppose with Copernicus, That the diurnal revolution is from the motion of the Earth, by which the Aequinoctial Circle is described about it. And as for the other two annual motions, they are the efficient cause of the Earths being carried about in the Ecliptick in such manner, as that its Axis is alwayes kept parallel to it self. Which parallelisme was for this reason introduced, lest by the Earths annual revolution, its Poles should seem to be necessarily carried about the Sunne, contrary to experience. I have (in the 10th Artic. of the th Chap.) demonstrated from the supposition of Simple Circular Motion in the Sun, that the Earth is so carried about the Sunne, as that its Axis is thereby kept always parallel to it self. Wherefore, from these two supposed motions in the Sunne, the one Simple Circular Motion; the other Circular Motion about its owns Center, it may be demonstrated, that the Year hath both the same variations of Dayes and Nights, as have been demonstrated by Copernicus.
For if the Circle abcd (in the 3d Figure) be the Ecliptick, whose Center is e, and Diameter aec; and the Earth be placed in a, & the Sunne be moved in the little Circle fghi, namely, according to the order f, g, h & i, it hath been demonstrated, that a Body placed in a, will be moved in the same order through the points of the Ecliptick a, b, c & d, and will alwayes keep its Axis parallel to its self.
But if (as I have supposed) the Earth also be moved with Simple Circular Motion in a plain that passeth through a, cutting the plain of the Ecliptick so, as that the common section of both the plains be in ac, thus also the Axis of the Earth will be kept alwayes parallel to it self. For let the Center of the Earth be moved about in the Circumference of the Epicycle whose Diameter is lak, which is a part of the straight line lac. Therefore lak the Diameter of the Epicycle, passing through the Center of the Earth, will be in the plain of the Ecliptick. Wherefore seeing that by reason of the Earths Simple Motion both in the Ecliptick and in its Epicycle, the straight line lak is kept alwayes parallel to it self, every other straight line also taken in the Body of the Earth, and consequently its Axis, will in like manner be kept alwayes parallel to it self; so that in what part soever of the Ecliptick the Center of the Epicycle be found, and in what part soever of the Epicycle the Center of the Earth be found at the same time, the Axis of the Earth will be parallel to the place where the same Axis would have been, if the Center of the Earth had never gone out of the Ecliptick.
Now as I have demonstrated the simple annual motion of the Earth from the supposition of Simple Motion in the Sunne; so from the supposition of Simple Motion in the Earth may be demonstrated the monethly Simple Motion of the Moon. For if the names be but changed the Demonstration will be the same, and therefore need not be repeated.
7 That which makes this supposition of the Sunnes Simple Motion in the Epicycle fghi probable, is First, that the Periods of all the Planets are not onely described about the Sunne, but so described, as that they are al contained within the Zodiack, that is to say, within the latitude of about 16 degrees; for the cause of this seems to depend upon some power in the Sunne, especially in that part of the Sunne which respects the Zodiack. Secondly, that in the whole copasse of the heavens there appears no other Body, from which the cause of this Phaenomenon can in probability be derived. Besides, I could not imagine, that so many and such various motions of the Planets should have no dependance at all upon one another. But by supposing motive power in the Sunne, we suppose motion also; for power to move, without motion, is no power at all. I have therefore supposed that there is in the Sunne for the governing of the primary Planets, and in the Earth for the governing of the Moon, such motion, as being received by the primary Planets and by the Moon, makes them necessarily appear to us in such manner as we see them; Whereas, that circular motion (which is commonly attributed to them) about a fixed Axis, (which is called Conversion) being a motion of their parts onely, and not of their whole Bodies, is insufficient to salve their Appearances. For seeing whatsoever is so moved, hath no endeavour at all towards those parts which are without the circle, they have no power to propagate any endeavour to such Bodies as are placed without it. And as for them that suppose this may be done by Magnetical Virtue, or by incorporeall and immateriall Species, they suppose no naturall cause; nay no cause at all. For there is no such thing as an Incorporeal Movent; and Magnetical Virtue is a thing altogether unknown; and whensoever it shall be known, it will be found to be a motion of Body. It remaines therefore, that if the primary Planets be carried about by the Sunne, and the Moon by the Earth, they have the simple circular motions of the Sunne and the Earth for the causes of their circulations. Otherwise, if they be not carried about by the Sunne and the Earth, but that every Planet hath been moved as it is now moved ever since it was made, there will be of their motions no cause naturall. For either these motions were concreated with their Bodies, and their cause is supernatural; or they are coeternal with them, and so they have no cause at all. For whatsoever is Eternall was never generated.
I may add besides, to confirme the probability of this simple motion, that allmost all learned men are now of the same opinion with Copernicus concerning the parallelisme of the Axis of the Earth, it seemed to me to be more agreeable to truth, or at least more handsome, that it should be caused by simple Circular Motion alone, than by two motions, one in the Ecliptick, and the other about the Earths own Axis the contrary way, neither of them Simple, nor either of them such as might be produced by any motion of the Sunne. I thought best therefore to retain this Hypothesis of Simple Motion; and from it to derive the causes of as many of the Phaenomena as I could, and to let such alone as I could not deduce frm thence.
It will perhaps be objected, that although by this supposition the reason may be given of the Parallelisme of the Axis of the Earth, and of many other Appearances; nevertheless, seeing it is done by placing the Body of the Sunne in the Center of that Orbe which the Earth describes with its annual motion, the supposition it self is false, because this annual Orbe is excentrique to the Sunne. In the first place therefore let us examine what that Excentricity is, and whence it proceeds.
8 Let the annual Circle of the Earth abcd (in the same 3d figure) be divided into four equal parts by the straight lines ac & bd, cutting one another in the Center e; and let a be the beginning of Libra, b of Capricorn, c of Aries, and d of Cancer; and let the whole Orbe abcd be understood (according to Copernicus) to have every way so great distance from the Zodiack of the fixed Starres, that it be in comparison with it but as a point. Let the Earth be now supposed to be in the beginning of Libra at a. The Sunne therefore will appear in the beginning of Aries at c. Wherefore if the Earth be moved from a to b, the apparent motion of the Sunne will be from c to the beginning of Cancer in d; and the Earth being moved forwards from b to c, the Sunne also will appear to be moved forwards to the beginning of Libra in a; Wherefore cda will be the Summer Arch, and the Winter Arch will be abc. Now in the time of the Suns apparent motion in the Summer Arch, there are numbred 186¾ dayes; and consequently the Earth makes in the same time the same number of diurnal conversions in the Arch abc; and therefore the Earth in its motion through the Arch cda will make onely 178½ diurnal conversions, Wherefore the Arch a b c ought to be greater then the Arch c d a by 8¼ dayes, that is to say, by almost so many degrees. Let the Arch a r, as also c s, be each of them an Arch of two degrees and 1/16. Wherefore the Arch r b s will be greater then the Semicircle a b c by 4 degrees and ⅛, and greater then the Arch s d r by 8 degrees and ¼. The Equinoxes therefore will be in the points r & s; and therefore also when the Earth is in r, the Sunne will appear in s. Wherefore the true place of the Sunne will be in t, that is to say, without the Center of the Earths annual motion by the quantity of the Sine of the Arch a r, or the Sine of two degrees and 16 minutes. Now this Sine, putting 100000 for the Radius, will be neer 3580 parts thereof. And so munh is the Excentricity of the Earths annual motion, provided that that motion be in a perfect circle; and s & r are the Equinoctial points; and the straight lines s r & c a produced both wayes till they reach the Zodiack of the fixed Starres, wil fall stil upon the same fixed Starres, because the whole Orbe a b c d is supposed to have no magnitude at all in respect of the great distance of the fixed Starres.
Supposing now the Sun to be in c, it remains that I shew the cause why the Earth is neerer to the Sunne, when in its annual motion it is found to be in d, then when it is in b. And I take the cause to be this. When the Earth is in the beginning of Capricorn at b, the Sunne appears in the beginning of Cancer at d; & then is the midst of Summer. But in the midst of Summer, the Northern parts of the Earth are towards the Sunne, which is almost all dry land, containing all Europe, and much the greatest part of Asia and America. But when the Earth is in the beginning of Cancer at d, it is the midst of Winter, and that part of the Earth is towards the Sunne, which contains those great Seas called the South Sea and the Indian Sea, which are of farre greater extent then all the dry Land in that Hemisphere. Wherefore (by the last Article of the 21 Chapter) when the Earth is in d, it will come neerer to its first Movent, that is, to the Sunne which is in t; that is to say, the Earth is neerer to the Sunne in the midst of Winter when it is in d, then in the midst of Summer when it is b; and therefore during the Winter the Sunne is in its Perigaeum, and in its Apogaeum during the Summer. And thus I have shewn a possible cause of the Excentricity of the Earth; which was to be done.
I am therefore of Keplers opinion, in this, that he attributes the Excentricity of the Earth to the difference of the parts thereof, and supposes one part to be affected, and another disaffected to the Sunne. And I dissent from him in this, that he thinks it to be by Magnetick virtue, and that this Magnetick virtue, or attraction and thrusting back of the Earth is wrought by immateriate Species; which cannot be; because nothing can give motion, but a Body moved and contiguous. For if those Bodies be not moved which are contiguous to a Body unmoved, how this Body should begin to be moved is not imaginable; as has been demonstrated in the 7th Article of the 9th Chapter, and often inculcated in other places, to the end that Philosophers might at last abstain from the use of such unconceiveable connexions of words. I dissent also from him in this, that he says the similitude of Bodies is the cause of their mutual attraction, For if it were so, I see no reason why one Egg should not be attracted by another. If therefore one part of the Earth be more affected by the Sunne, then another part, it proceeds from this, that one part hath more water, the other more dry land. And from hence it is, (as I shewed above) that the Earth comes neerer to the Sunne when it shines upon that part where there is more water, then when it shines upon that where there is more dry Land.
9 This Excentricity of the Earth is the cause why the way of its annual motion is not a perfect Circle, but either an Elliptical, or almost an Elliptical line; as also why the Axis of the Earth is not kept exactly parallel to it self in all places, but onely in the Equinoctial points.
Now seeing I have said that the Moon is carried about by the Earth, in the same manner that the Earth is by the Sunne; and that the Earth goeth about the Sunne in such manner as that it shews sometimes one Hemisphere, sometimes the other to the Sunne; it remains to be enquired, why the Moon has alwayes one and the same face turned towards the Earth.
Suppose therefore the Sunne to be moved with Simple Motion in the little Circle f g h i, (in the fourth figure) whose Center is t; and let ♈ ♋ ♎ ♑ be the annuall Circle of the Earth; and a the beginning, of Libra. About the point a let the little Circle l k be described; and in it let the Center of the Earth be understood to be moved with Simple motion; and both the Sunne & the Earth to be moved according to the order of the Signes. Upon the Center a let the way of the Moon m n o p be described; and let q r be the Diameter of a Circle cutting the Globe of the Moon into two Hemispheres, whereof one is seen by us when the Moon is at the full, and the other is turned from us.
The Diameter therefore of the Moon q o r will be perpendicular to the Straight Line t a. Wherefore the Moon is carried by reason of the Motion of the Earth from o towards p. But by reason of the motion of the Sunne, if it were in p it would at the same time be carried from p towards o; and by these two contrary Movents the straight line q r will be turned about; and in a Quadrant of the Circle m n o p it will be turned so much as makes the fourth part of its whole conversion. Wherefore when the Moon is in p, q r will be parallel to the straight line m o. Secondly, when the Moon is in m, the straight line q r will by reason of the motion of the Earth be in m o. But by the working of the Suns motion upon it in the quadrant p m, to same q r will be turned so much as makes another quarter of its whole conversion. When therefore the Moon is in m, q r will be perpendicular to the straight line o m. By the same reason, when the Moon is in n, q r will be parallel to the straight line m o; and the Moon returning to o, the same q r will return to its first place; and the Body of the Moon will in one entire period make also one entire conversion upon her own Axis. In the making of which, it is manifest, that one and the same face of the Moon is always turned towards the Earth. And if any Diameter were taken in that little Circle, in which the Moon were supposed to be carried about with simple motion, the same effect would follow; for if there were no action from the Sun, every Diameter of the Moon would be carried about always parallel to it self. Wherefore I have given a possible cause, why one and the same face of the Moon is alwayes turned towards the Earth.
But it is to be noted, that when the Moon is without the Ecliptick, we do not alwayes see the same face precisely. For we see onely that part which is illuminated. But when the Moon is without the Ecliptick, that part which is towards us, is not exactly the same with that which is illuminated.
10 To these three simple motions, one of the Sunne, another of the Moon, and the third of the Earth, in their own little Circles f g h i, l k, & q r, together with the Diurnal conversion of the Earth (by which conversion all things that adhere to its superficies are necessarily carried about with it) may be referred the three Phaenomena concerning the Tides of the Ocean. Whereof the first, is the alternate elevation and depression of the Water at the Shores, twice in the space of 24 houres and neer upon 52 minutes; for so it has constantly continued in all ages. The second, that at the New and Full Moons the elevations of the Water are greater, then at other times between. And the third, that when the Sunne is in the Equinoctial, they are yet greater then at any other time. For the salving of which Phaenomena, we have already the foure abovementioned Motions; to which I assume also this, that the part of the Earth which is called America, being higher then the Water, and extended almost the space of a whole Semicircle from North to South, gives a stop to the motion of the Water. This being granted,
In the same 4th figure, where l b k c is supposed to be in the plain of the Moons monethly motion, let the little Circle l d k e be described about the same Center a in the plain of the Equinoctial. This Circle therefore will decline from the Circle l b k c in an angle of almost 28 degrees and ½ (for the greatest declination of the Ecliptick is 23½, to which adding 5 for the greatest declination of the Moon from the Ecliptick, the summe wil be 28 degrees and ½). Seeing now the Waters which are under the Circle of the Moons course, are (by reason of the Earths Simple Motion in the plain of the same Circle) moved together with the Earth, (that is to say, together with their own bottoms) neither out-going nor out-gone; if we add the Diurnal motion, by which the other Waters which are under the Equinoctial are moved in the same order; and consider withall that the Circles of the Moon and of the Equinoctial intersect one another; it will be manifest, that both those Waters, which are under the Circle of the Moon, and under the Equinoctiall, will runne together under the Equinoctial; and consequently, that their Motion will not onely be swifter then the ground that carries them; but also that the waters themselves will have greater elevation whensoever the Earth is in the Equinoctial. Wherefore, whatsoever the cause of the Tides may be, this may be the cause of their augmentation at that time.
Againe, seeing I have supposed the Moon to be carried about by the simple motion of the Earth in the little circle lbkc; and demonstrated (at the 4 article of the 21 chapter) that whatsoever is moved by a Movent that hath simple motion, will be moved allwayes with the same velocity; it follows, that the center of the Earth will be carried in the circumference lbkc with the same velocity, with which the Moon is carried in the circumference mnop. Wherefore the time in which the Moon is carried about in mnop, is to the time in which the Earth is carried about in lbkc, as one circumference to the other, that is, as ao to ak. But ao is observed to be to the Semidiameter of the Earth as 59 to 1; and therefore the Earth (if ak be put for its Semidiameter) will make 59 revolutions in lbkc, in the time that the Moon makes one monthly circuit in mnop. But the Moon makes her monthly circuit in little more then 29 dayes. Wherefore the Earth shal make its circuit in the circumference lbkc in 12 hours and a little more, namely about 26 minutes more; that is to say, it shall make two circuits in 24 hours and allmost 52 minutes; which is observed to be the time between the high water of one day and the high water of the day following. Now the course of the waters being hindered by the southern part of America, their motion will be interrupted there; and consequently, they will be elevated in those places, and sink down again by their own waight, twice in the space of 24 hours and 52 minutes. And thus I have given a possible cause of the diurnall reciprocation of the Ocean.
Now from this swelling of the Ocean in those parts of the Earth, proceed the Flowings and Ebbings in the Atlantick, Spanish, Brittish and German Seas; which though they have their set times, yet upon severall Shores they happen at severall hours of the day; and they receive some augmentation from the North, by reason that the shores of China and Tartaria, hindering the generall course of the waters, makes them swell there and discharge themselves in part through the straight of Anian into the Northern Ocean, and so into the German Sea.
As for the Spring Tides which happen at the time of the New & Full Moons, they are caused by that simple motion which at the beginning I supposed to be allwayes in the Moone. For as, when I shewed the cause of the Excentricity of the Earth, I derived the elevation of the waters from the simple motion of the Sunne; so the same may here be derived from the simple motion of the Moon. For though from the generation of Clouds, there appeare in the Sunne a more manifest power of elevating the waters, then in the Moon; yet the power of encreasing moisture in Vegetables and living creatures appears more manifestly in the Moon then in the Sunne; which may perhaps proceed from this, that the Sunne raiseth up greater, and the Moon lesser drops of water. Neverthelesse, it is more likely, and more agreeable to common observation, that Raine is raised not only by the Sunne but also by the Moon; for allmost all men expect change of weather at the time of the Conjunctions of the Sunne and Moon with one another, and with the Earth, more then in the time of their Quarters.
In the last place, the cause why the Spring Tides are greater at the time of the Aequinoxes, hath been already sufficiently declared in this Article, where I have demonstrated, that the two Motions of the Earth, namely, its Simple Motion in the little Circle lbkc, and its Diurnal motion in ldke, cause necessarily a greater elevation of Waters when the Sunne is about the Aequinoxes, then when he is in other places. I have therefore given possible causes of the Phaenomenon of the Flowing and Ebbing of the Ocean.
11 As for the explication of the yearly Praecession of the Aequinoctial points, we must remember, that (as I have already shewn) the annual motion of the Earth is not in the Circumference of a Circle, but of an Ellipsis, or a line not considerably different from that of an Ellipsis. In the first place therefore this Elliptical line is to be described.
Let the Ecliptick ♎ ♑ ♈ ♋ (in the 5th figure) be divided into four equal parts by the two straight lines ab and ♑ ♋, cutting one another at right angles in the Center c; and taking the Arch bd of 2 degrees and 16 minutes, let the straight line de be drawn parallel to ab and cutting ♑ ♋ in f; which being done, the Excentricity of the Earth will be cf. Seeing therefore the annual motion of the Earth is in the Circumference of an Ellipsis, of which ♑ ♋ is the greater Axis, ab cannot be the lesser Axis; for ab and ♑ ♋ are equal. Wherefore the Earth passing through a & b, will either pass above ♑, as through g, or passing through ♑, will fall between c and a; it is no matter which. Let it pass therefore through g; and let gl be taken equal to the straight line ♑ ♋; and dividing gl equally in i, gi will be equal to ♑ ♋, & il equal to f ♋; and consequently the point i will cut the Excentricity cf into two equal parts; and taking ih equal to if, hi will be the whole Excentricity. If now a straight line (namely, the line ♎ i ♈) be drawn through i parallel to the straight lines ab and ed, the way of the Sunne in Summer (namely, the Arch ♎ g ♈) will be greater then his way in Winter by 8 degrees and ¼. Wherefore the true Aequinoxes wil be in the straight line ♎ i ♈; and therefore the Ellipsis of the Earths annual motion will not pass through a, g, b & l; but through ♎, g, ♈ & l. Wherfore the annual motion of the Earth is in the Ellipsis ♎ g ♈ l, and cannot be (the Excentricity being salved) in any other line. And this perhaps is the reason, why Kepler, against the opinion of all the Astronomers of former time, thought fit to bisect the Excentricity of the Earth, or (according to the Ancients) of the Sunne, not by diminishing the quantity of the same Excentricity, because the true measure of that quantity, is the difference by which the Summer Arch exceeds the Winter Arch, but by taking for the Center of the Ecliptick of the great Orbe the point c neerer to f, & so placing the whole great Orbe as much neerer to the Ecliptick of the fixed Stars towards ♋, as is the distance between c & i. For seeing the whole great Orbe is but as a point in respect of the immense distance of the fixed Starres, the two straight lines ♎ ♈ and ab being produced both wayes to the beginnings of Aries and Libra, will fall upon the same points of the Sphere of the fixed Stars. Let therefore the Diameter of the Earth mn be in the plain of the Earths annual motion. If now the Earth be moved by the Sunnes simple motion in the Circumference of the Ecliptick about the Center i, this Diameter will bee kept alwayes parallel to itself and to the straight line gl. But seeing the Earth is moved in the Circumference of an Ellipsis without the Ecliptick, the point n, whilst it passeth through ♎ ♑ ♈ will go in a lesser Circumference then the point m; and consequently as soon as ever it begins to be moved, it will lose its parallelisme with the straight line ♑ ♋ so that mn produced will at last cut the straight line gl produced. And contrarily, as soon as mn is past ♈, (the Earth making its way in the internal Ellipticall line ♈ l ♎) the same mn produced towards m, will cut lg produced. And when the Earth hath allmost finished its whole circumference, the same mn shall againe make a right angle with a line drawn from the center i, a little short of the point from which the Earth began its motion. And there the next yeare shall be one of the Aequinoctial points, namely, neer the end of ♍; the other shall be opposite to it neer the end of ♓. And thus the points in which the Days and Nights are made equall, doe every year fall back; but with so slow a motion, that in a whole year, it makes but 51 first minutes. And this relapse being contrary to the order of the Signes, is commonly called the Praecession of the Aequinoxes. Of which I have from my former Suppositions deduced a possible cause; which was to be done.
According to what I have said concerning the cause of the Excentricity of the Earth; and according to Kepler, who for the cause thereof supposeth one part of the Earth to be affected to the Sunne the other part to be disaffected, the Apogaeum & Perigaeum of the Sunne should be moved every year in the same order, and with the same velocity, with which the Aequinoctiall points are moved; and their distance from them should allwayes be the quadrant of a circle; which seems to be otherwise. For Astronomers say, that the Aequinoxes are now, the one about 28 degrees gone back from the first Star of Aries, the other as much from the beginning of Libra. So that the Apogaeum of the Sunne, or the Aphelium of the Earth ought to be about the 28th degree of Cancer; but it is reckoned to be in the 7th degree. Seeing therefore we have not sufficient evidence of the (that so it is), it is in vaine to seek for the (why it is so). Wherefore, as long as the motion of the Apogaeum is not observable by reason of the slownesse thereof; and as long as it remaiues doubtful whether their distance from the Aequinoctiall points be more or lesse then a quadrant precisely; so long it may be lawfull for me to thinke they proceed both of them with equall velocity.
Also, I doe not at all meddle with the causes of the Excentricities of Saturne, Jupiter, Mars, and Mercury. Neverthelesse, seeing the Excentricity of the Earth may (as I have shewne) be caused by the unlike constitution of the several parts of the Earth which are alternately turned towards the Sunne, it is credible also that like effects may be produced in these other Planets from their having their Superficies of unlike parts.
And this is all I shall say concerning Sidereal Philosophy. And though the causes I have here supposed be not the true causes of these Phaenomena, yet I have demonstrated that they are sufficient to produce them, according to what I at first propounded.
CHAP. XXVII. OF LIGHT, HEAT, AND OF COLOURS.
1 OF THE immense Magnitude of some Bodies, and the unspeakable Littleness of others.
2 Of the cause of the Light of the Sun.
3 How Light heateth.
4 The generation of Fire from the Sunne.
5 The generation of Fire from Collision.
6 The cause of Light in Glow-wormes, Rotten Wood, and the Bolonian Stone.
7 The cause of Light in the concussion of Sea-water.
8 The cause of Flame, Sparks, and Colliquation.
9 The cause why wet Hay sometimes burns of its own accord. Also the cause of Lightning.
10 The cause of the force of Gunpowder; and what is to be ascribed to the Coals, what to the Brimstone, and what to the Nitre.
11 How Heat is caused by Attrition.
12 The distinction of Light into First, Second, &c.
13 The causes of the Colours we see in looking through a Prisma of Glass, namely, of Red, Yellow, Blue and Violet Colour.
14 Why the Moon and the Starres appear redder in the Horizon then in the midst of the Heaven.
15 The cause of Whiteness.
16 The cause of Blackness.
1 BESIDES the Starres (of which I have spoken in the last Chapt.), whatsoever other Bodies there be in the World, they may be all comprehended under the name of Intersidereal Bodies. And these I have already supposed to be either the most fluid Aether, or such Bodies whose parts have some degree of cohaesion. Now these differ from one another in their several Consistencies, Magnitudes, Motions and Figures. In Consistency I suppose some Bodies to be Harder, others Softer through all the several degrees of Tenacity. In Magnitude, some to be Greater, others Less, and many unspeakably Little. For we must remember that by the Understanding, Quantity is divisible into divisibles perpetually. And therefore if a man could do as much with his hands as he can with his Understanding, he would be able to take from any given magnitude, a part which should be less then any other magnitude given. But the Omnipotent Creator of the World can actually from a part of any thing take another par, as farre as we by our Understanding can conceive the same to be divisible. Wherefore there is no impossible Smalness of Bodies. And what hinders but that we may think this likely? For we know there are some living Creatures so small that we can scarce see their whole Bodies. Yet even these have their young ones, their little Veins, and other Vessels, and their Eyes so smal, as that no Microscope can make them visible. So that we cannot suppose any magnitude so little, but that our very supposition is actually exceeded by Nature. Besides, there are now such Microscopes commonly made, that the things we see with them appear a hundred thousand times bigger, then they would do if we looked upon them with our bare Eyes. Nor is there any doubt but that by augmenting the power of these Microscopes (for it may be augmented as long as neither Matter nor the hands of Workmen are wanting) every one of those hundred thousandth parts might yet appear a hundred thousand times geater then they did before. Neither is the Smalness of some Bodies to be more admired, then the vast Greatness of others. For it belongs to the same infinite Power, as well to augment infinitely, as infinitely to diminish; To make the great Orbe, (namely, that whose Radius reacheth from the Earth to the Sunne) but as a point in respect of the distance between the Sunne and the fixed Starres; and on the contrary, to make a Body so little, as to be in the same proportion less then any other visible Body, proceeds equally from one and the same Authour of Nature. But this of the immense distance of the fixed Starres (which for a long time was accounted an incredible thing) is now believed by almost all the Learned. Why then should not that other of the smalness of some Bodies, become credible at some time or other? For the Majesty of God appears no less in small things then in great; and as it exceedeth humane sense in the immense greatness of the Universe; so also it doth in the smalness of the parts thereof. Nor are the first Elements of Compositions, nor the first Beginnings of Actions, nor the first Moments of Times more credible, then that which is now believed of the vast distance of the fixed Starres.
Some things are acknowledged by mortal men to be very Great, though Finite, as seeing them to be such. They acknowledge also, that some things which they do not see, may be of infinite magnitude. But they are not presently, nor without great study perswaded that there is any Mean between Infinite & the Greatest of those things which either they see or imagine. Nevertheless, when after meditation & contemplation, many things which we wondred at before are now grown more familiar to us, we then believe them, and transferre our admiration from the Creatures to the Creator. But how little soever some Bodies may be; yet I will not suppose their quantity to be less, then is requisite for the salving of the Phaenomena. And in like manner I shall suppose their motion, namely, their Velocity and Slowness, and the Variety of their Figures, to be onely such as the explication of their natural causes requires. And lastly, I suppose, that the parts of the pure Aether (as if it were the First Matter) have no motion at all, but what they receive from Bodies which float in them, and are not themselves fluid.
2 Having laid these Grounds, let us come to speake of Causes, and in the first place let us inquire what may be the cause of the Light of the Sunne. Seeing therefore the Body of the Sunne doth by its simple circular motion thrust away the ambient aethereall substance sometimes one way sometimes another, so that those parts which are next the Sunne being moved by it, doe propagate that motion to the next remote parts, and these to the next, and so on continually; it must needs be, that notwithstanding any distance, the foremost part of the Eie will at last be pressed; and by the pressure of that part, the motion will be propagated to the innermost part of the Organ of Sight, namely to the Heart; and from the reaction of the Heart, there will proceed an endeavour back by the same way, ending in the endeavour outwards of the Coat of the Eie called the Retina. But this endeavour outwards (as has been defined in the 25 chapter) is the thing which is called Light, or the Phantasme of a Lucid Body. For it is by reason of this Phantasme that an Object is called Lucid. Wherefore we have a possible cause of the Light of the Sunne; which I undertook to find.
3 The generation of the Light of the Sunne is accompanied with the generation of Heat. Now every man knowes what Heat is in himselfe, by feeling it when he growes Hot; but what it is in other things he knowes onely by ratiocination. For it is one thing to grow Hot, and another thing to Heat, or make Hot. And therefore though we perceive that the Fire or the Sunne Heateth, yet we doe not perceive that it is it selfe Hot. That other living creatures whilest they make other things Hot, are Hot themselves, we inferre by reasoning from the like sense in our selves. But this is not a necessary inference. For though it may truly be said of living Creatures, that They Heat, therefore they are themselves Hot; yet it cannot from hence be truly inferred, that Fire Heateth, therefore it is it selfe Hot; no more then this, Fire causeth Pain, therefore it is it self in Pain. Wherefore that is onely and properly called Hot, which when we feel, we are necessarily Hot.
Now when we grow Hot, we find that our Spirits and Blood, and whatsoever is fluid within us, is called out from the internall to the externall parts of our Bodies, more or lesse, according to the degree of the Heat; and that our Skin swelleth. He therefore that can give a possible cause of this Evocation and Swelling, and such as agreeth with the rest of the Phaenomena of Heat; may be thought to have given the cause of the Heat of the Sunne.
It hath been shewn (in the 5 article of the 21 chapter) that the fluid Medium which we call the Aire, is so moved by the simple circular motion of the Sunne, as that all its parts, even the least, do perpetually change places with one another; which change of places is that which there I called Fermentation. From this Fermentation of the Aire, I have (in the 8 article of the last chapter) demonstrated, that the water may be drawn up into the clouds.
And I shall now shew, that the fluid parts may in like manner by the same Fermentation be drawn out from the internall to the externall parts of our Bodies. For seeing that wheresoever the fluid Medium is contiguous to the Body of any living creature, there the parts of that Medium are by perpetuall change of place separated from one another; the contiguous parts of the living creature must of necessity endeavour to enter into the spaces of the separated parts. For otherwise those parts (supposing there is no Vacuum) would have no place to go into. And therefore that which is most fluid and separable in the parts of the living creature which are contiguous to the Medium, will go first out; and into the place thereof will succeed such other parts as can most easily transpire through the poes of the skin. And from hence it is necessary, that the rest of the parts which are not separated, must all together be moved outwards, for the keeping of all places full. But this motion outwards of all parts together must of necessity press those parts of the ambient Aire which are ready to leave their places; and therefore all the parts of the Body endeavouring at once that way, makes the Body swell. Wherefore a possible cause is given of Heat from the Sunne; which was to be done.
4 We have now seen how Light and Heat are generated; Heat by the simple motion of the Medium, making the parts perpetually change places with one another; and Light by this, that by the same simple motion Action is propagated in a straight line. But when a Body hath its parts so moved, that it sensibly both Heats and Shines at the same time, then it is, that we say Fire is generated.
Now by Fire I do not understand a Body distinct from matter combustible or glowing, as Wood or Iron, but the matter it self, not simply and always, but then onely when it shineth and heateth. He therefore that renders a cause possible, and agreeable to the rest of the Phaenomena, namely, whence and from what action both the Shining and Heating proceed, may be thought to have given a possible cause of the generation of Fire.
Let therefore ABC (in the first Figure) be a Sphere, or the portion of a Sphere, whose Center is D; and let it be transparent and homogeneous, as Cristal, Glass, or Water, and objected to the Sunne. Wherefore the foremost part ABC, will by the simple motion of the Sunne, by which it thrusts forwards the Medium be wrought upon by the Sun-beams in the straight lines EA, FB and GC; which straight lines may in respect of the great distance of the Sunne be taken for parallels. And seeing the Medium within the Sphere is thicker then the Medium without it, those Beams will be refracted towards their perpendiculars. Let the straight lines EA and GC be produced till they cut the Sphere in H and I; and drawing the perpendiculars AD and CD, the refracted Beams EA and GC will of necessity fall, the one between AH and AD, the other between CI and CD. Let those refracted Beams be AK and CL. And again, let the lines DKM & DLN be drawn perendicular to the Sphere; and let AK and CL be produced till they meet with the straight line BD produced in O. Seeing therefore the Medium within the Sphere is thicker then that without it, the refracted line AK will recede further from the perpendicular KM, then KO will recede from the same. Wherefore KO will fall between the refracted line and the perpendicular. Let therefore the refracted line be KP, cutting FO in P, and for the same reason the straight line LP will be the refracted line of the straight line CL. Wherfore, seeing the Beams are nothing else but the Wayes in which the motion is propagated, the motion about P will be so much more vehement then the motion about ABC, by how much the base of the portion ABC is greater then the base of a like portion in the Sphere whose Center is P, and whose magnitude is equal to that of the little Circle about P, which comprehendeth all the Beams that are propagated from ABC; and this Sphere being much less then the Sphere ABC, the parts of the Medium, that is, of the Aire about P, will change places with one another with much greater celerity, then those about ABC. If therefore any matter Combustible, that is to say, such as may be easily dissipated, be placed in P, the parts of that matter (if the proportion be great enough between AC and a like portion of the little circle about P) wil be freed from their mutual cohaesion, and being separated will acquire simple motion. But vehement simple motion generates in the beholder a Phantasm of Lucid and Hot, as I have before deonstrated of the simple motion of the Sunne; and therefore the combustible matter which is placed in P will be made Lucid and Hot, that is to say, will be Fire. Wherefore I have rendered a possible cause of Fire; which was to be done.
5 From the manner by which the Sunne generateth Fire, it is easy to explaine the manner by which Fire may be generated by the collision of two Flints. For by that Collision, some of those particles of which the stone is compacted, are violently separated and thrown off; and being withall swiftly turned round, the Eie is moved by them, as it is in the generation of Light by the Sunne. Wherefore they shine; and falling upon matter which is already halfe dissipated, such as is Tinder, they throughly dissipate the parts thereof, and make them turn round. From whence (as I have newly shewn) Light and Heat, that is to say, Fire is generated.
6 The shining of Glow-worms, some kinds of Rotten Wood, and of a kinde of stone made at Bolognia, may have one common cause, namely the exposing of them to the hot Sunne. We finde by experience that the Bolonian stone shines not unless it be so exposed; and after it has been exposed it shines but for a little time, namely, as long as it retains a certain degree of heat. And the cause may be, that the parts of which it is made, may together with heat have Simple Motion imprinted in them by the Sunne. Which if it be so, it is necessary, that it shine in the dark, as long as there is sufficient heat in it; but this ceasing, it will shine no longer. Also we find by experience, that in the Glow-worm there is a certain thick humour, like the Cristalline humour of the Eie; which if it be taken out, and held long enough in ones fingers, and then be carried into the dark, it will shine by reason of the warmth it received from the fingers; but as soon as it is cold, it will cease shining. From whence therefore can these creatures have their Light, but from lying all day in the Sun-shine, in the hottest time of Summer? In the same manner, Rotten Wood, except it grow rotten in the Sunshine, or be afterwards long enough exposed to the Sunne, will not shine. That this doth not happen in every Worm, nor in all kinds of Rotten Wood, nor in all Calcined Stones, the cause may be, that the parts of which those Bodies are made, are different both in motion and figure from the parts of Bodies of other kinds.
7 Also the Sea-water shineth when it is either dashed with the strokes of Oares, or when a Ship in its course breaks strongly through it; but more or less according as the Winde blows from different points. The cause whereof may be this, that the particles of salt (though they never shine in the Salt-pits, where they are but slowly drawn up by the Sunne) being here beaten up into the aire in greater quantities, and with more force, are withall made to turn round, and consequently to shine, though weakly. I have therefore given a possible cause of this Phaenomenon.
8 If such matter as is compounded of hard little Bodies, be set on fire, it must needs be, that as they flye out in greater or lesse quantities, the Flame which is made by them, will be greater or less. And if the aethereal or fluid part of that matter fly out together with them, their motion will be the Swifter, as it is in Wood, and other things which flame with a manifest mixture of Winde. When therefore these hard particles by their flying out, move the Eye strongly, they shine bright; and a great quantity of them flying out together, they make a great shining Body. For Flame being nothing but an aggregate of shining particles, the greater the aggregate is, the greater and more manifest will be the Flame. I have therefore shewn a possible cause of Flame. And from hence the cause appears evidently, why Glass is so easily and quickly melted by the small Flame of a candle blown, which will not be melted without blowing, but by a very strong Fire.
Now if from the same matter, there be a part broken off (namely such a part as consisteth of many of the small particles), of this is made a Spark. For from the breaking off, it hath a violent turning round; and from hence it shines. But though from this matter, there fly neither Flame nor Sparks; yet some of the smallest parts of it may be carried out as farre as to the Superficies, and remain there, as Ashes; the parts whereof are so extremely small, that it cannot any longer be doubted how farre Nature may proceed in Dividing.
Lastly, though by the application of fire to this matter, there fly little or nothing from it, yet there will be in the parts an endeavour to Simple motion; by which the whole Body will either be Melted, or (which is a degree of Melting) Softned. For all Motion has some effect upon all Matter whatsoever (as has been shewn at the 3d Article of the 16th Chapter). Now if it be softned to such a degree, as that the stubborness of the parts be exceeded by their gravity, then we say it is Melted; otherwise, Softned, and made Pliant and Ductile.
Again, the matter having in it some particles hard, others aethereal or watery, if by the application of fire these later be called out, the former will thereby come to a more full contact with one another; and consequently will not be so easily separated; that is to say, the whole Body will be made Harder. And this may be the cause why the same Fire makes some things Soft, others Hard.
9 It is known by experience, that if Hay be laid wet together in a heap, it will after a time begin to smoke, and then burn as it were of it self. The cause whereof seems to be this, that in the Aire which is enclosed within the Hay, there are those little Bodies, which (as I have supposed) are moved freely with simple Motion. But this Motion being by degrees hindred more and more by the descending moisture, which at the last fils and stops all the passages, the thinner parts of the Aire ascend by penetrating the water; and those hard little Bod being so thrust together that they touch and press one another, acquire stronger motion, till at last by the increased strength of this motion the watery parts are first driven outwards, from whence appears Vapour; and by the continued increase of this motion, the smallest particles of the dryed Hay are forced out, and recovering their natural simple Motion, they grow Hot and Shine, that is to say, they are set on Fire.
The same also may be the cause of Lightning; which happens in the hottest time of the yeare, when the water is raised up in greatest quantity, and carried highest. For after the first Clouds are raised, others after others follow them; and being congeled above, they happen (whilest some of them ascend and others descend) to fall upon another in such manner, as that in some places all their parts are joyned together, in others they leave hollow Spaces between them; aud into these spaces (the aethereall parts being forced out by the compressure of the Clouds) many of the harder little Bodies are so pent together, as that they have not the liberty of such motion as is naturall to the Aire. Wherefore their endeavour growes more vehement, till at last they force their way through the Clouds, sometimes in one place, sometimes in another; and breaking through with great noise, they move the aire violently, & striking our Eies generate Light, that is to say, they Shine. And this Shining is that we call Lightning.
10 The most common Phaenomenon proceeding from Fire, and yet the most admirable of all others, is the force of Gunpowder fired; which being compounded of Niter, Brimstone and Coles beaten small, hath from the Coles its first taking fire; from the Brimstone its nourishment and flame, that is to say, Light and motion; and from the Niter the vehemence of both. Now if a piece of Niter, (before it is beaten) be laid upon a burning Cole, first it melts, and like water quencheth that part of the Cole it toucheth. Then Vapor or Aire flying out where the Cole and Niter joyne, bloweth the Cole with great swiftnesse and vehemence on all sides. And from hence it comes to passe, that by two contrary motions (the one, of the particles which go out of the burning Cole, the other, of those of the aethereall and watery substance of the Niter) is generated that vehement Motion and Inflammation. And lastly, when there is no more action from the Niter (that is to say, when the volatile parts of the Niter are flown out) there is found about the sides a certain white substance, which being thrown again into the fire, will grow red hot again, but will not be dissipated, at least, unlesse the fire be augmented. If now a possible cause of this be found out, the same will also be a possible cause why a grain of Gunpowder set on fire doth expand it selfe with such vehement motion, and Shine. And it may be caused in this manner.
Let the particles of which Niter consisteth, be supposed to be some of them hard, others watery, and the rest aethereall. Also let the hard particles be supposed to be spherically hollow, like small bubbles, so that many of them growing together may constitute a Body whose little cavernes are filled with a substance which is either watery, or aethereal, or both. As soon therefore as the hard particles are dissipated, the watery and aethereal particles will necessarily fly out; and as they fly, of necessity blow strongly the burning Coles and Brimstone which are mingled together; whereupon there will follow a great expansion of Light, with vehement flame, and a violent dissipation of the particles of the Niter, the Brimstone and the Coles. Wherefore I have given a possible cause of the force of fired Gunpowder.
It is manifest from hence, that for the rendering of the cause why a bullet of lead or iron shot from a peece of Ordnance flies with so great velocity, there is no necessity to introduce such Rarefaction, as (by the common definition of it) makes the same Matter to have sometimes more, sometimes lesse Quantity; which is unconceiveable. For every thing is said to be greater or lesse, as it hath more or lesse Quantity. The violence with which a bullet is thrust out of a Gun, proceeds from the swiftnesse of the small particles of the fired Powder; at least it may proceed from that cause without the supposition of any Empty Space.
11 Besides, by the attrition or rubbing of one Body against another, as of Wood against Wood, we find that not only a certaine degree of Heat, but Fire it selfe is sometimes generated. For such motion, is the reciprocation of pressure, sometimes one way sometimes the other; and by this reciprocation, whatsoever is fluid in both the peeces of Wood, is forced hither and thither; and consequently, to an endeavour of getting out; and at last by breaking out makes Fire.
12 Now Light is distinguished into, First, Second, Third, and so on infinitely. And we call that First Light, which is in the first Lucid Bodie; as the Sunne, Fire, &c. Second, that which is in such Bodies as being not transparent are illuminated by the Sunne; as the Moon, a Wall, &c. and Third, that which is in Bodies not transparent but illuminated by Second Light, &c.
13 Colour is Light, but troubled Light; namely, such as is generated by perturbed motion; as shall be made manifest by the Red, Yellow, Blew and Purple which are generated by the interposition of a Diaphanous Prisma (whose opposite bases are triangular) between the Light and that which is enlightened.
For let there be a Prisma of Glasse, or of any other transparent matter which is of greater density then Aire; and let the triangle ABC be the base of this Prisma. Also let the straight line DE be the diameter of the Sunnes Body, having oblique position to the straight line AB; and let the Sunne-beames passe in the lines DA and EBC. And lastly let the straight lines DA and EC be produced indefinitely to F and G. Seeing therefore the straight line DA, by reason of the density of the Glasse is refracted towards the perpendicular; let the line refracted at the point A be the straight line AH. And againe, seeing the Medium below AC is thinner then that above it, the other refraction which will be made there, will diverge from the perpendicular. Let therefore this second refracted line be AI. Also let the same be done at the point C, by making the first refracted line to be CK, and the second CL. Seeing therefore the cause of the refraction in the point A of the straight line of AB, is the excess of the resistance of the Medium in AB above the resistance of the Aire, there must of necessity be reaction from the point A towards the point B; and consequently the Medium at A within the triangle ABC, will have its motion troubled; that is to say, the straight motion in AF and AH, will be mixed with the transverse motion between the same AF and AH, represented by the short transverse lines in the triangle AFH. Againe, seeing at the point A of the straight line AC, there is a second refraction from AH in AI, the motion of the Medium will againe be perturbed by reason of the transverse reaction from A towards C, represented likewise by the short transverse lines in the triangle AHI. And in the same manner there is a double perturbation represented by the transverse lines in the triangles CGK and CKL. But as for the light between AI and CG, it will not be perturbed; because if there were in all the points of the straight lines AB and AC, the same action which is in the points A and C, then the plaine of the triangle CGK would be every where coincident with the plaine of the triangle AFH; by which meanes all would appear alike between A and C. Besides, it is to be observed, that all the reaction at A, tends towards the illuminated parts which are between A and C, and consequently perturbeth the First Light. And on the contrary, that all the reaction at C tends towards the parts without the triangle, or without the Prisma ABC, where there is none but Second Light; and that the triangle AFH shewes that perturbation of Light which is made in the Glasse it selfe; as the triangle AHI shewes that perturbation of Light which is made below the Glasse. In like manner, that CGK shewes the perturbation of Light within the Glasse; and CKL that which is below the Glasse. From whence there are four divers motions, or four different illuminations or Colours; whose differences appear most manifestly to the Sense in a Prisma (whose base is an equilaterall triangle) when the Sunne-beames that passe through it are received upon a white paper. For the triangle AFH appears Red to the Sense; the triangle AHI Yellow; the triangle CGK Green, and approaching to Blew; and lastly the triangle CKL appears Purple. It is therefore evident, that when weak but First Light passeth through a more resisting diaphanous Body, as Glasse, the beames which fall upon it tranversly, make Rednesse; and when the same First Light is stronger, as it is in the thinner Medium below the straight line AC, the transverse beames make Yellownesse. Also when Second Light is strong, as it is in the triangle CGK (which is neerest to the First Light) the transverse beames make Greenesse; and when the same Second Light is weaker, as in the triangle CKL, they make a Purple colour.
14 From hence may be deduced a cause why the Moon and Starres appear bigger and redder neer the Horizon then in the Mid-heaven. For between the Eie and the apparent Horizon, there is more impure aire, such as is mingled with Watery and Earthy little Bodies, then is between the same Eie and the more elevated part of Heaven. But Vision is made by Beames which constitute a Cone, whose base, if we look upon the Moon, is the Moons Face, and whose vertex is in the Eie; and therefore many beams from the Moon must needs fall upon little Bodies that are without the Visual Cone, and be by them reflected to the Eie. But these reflected beams tend all in lines which are transverse to the Visual Cone, and make at the Eie an angle which is greater then the angle of the Cone. Wherefore the Moon appeares greater in the Horizon, then when she is more elevated. And because those reflected beames go transversely, there will be generated (by the last article) Rednesse. A possible cause therefore is shewne, why the Moon, as also the Starres appear Greater and Redder in the Horizon, then in the midst of heaven. The same also may be the cause why the Sunne appears in the Horizon, Greater, and of a colour more degenerating to Yellow, then when he is higher elevated. For the reflection from the little Bodies between, and the transverse motion of the Medium are still the same. But the Light of the Sunne is much stronger then that of the Moon; and therefore (by the last article) his Splendor must needs by this perturbation degenerate into Yellownesse.
But for the generation of these four colours, it is not necessary that the figure of the Glass be a Prisma; for if it were Spherical it would doe the same. For in a Sphere the Sunne-beames are twice refracted and twice reflected. And this being observed by Des Cartes; and with all that a Rainebow never appeares but when it rains; as also, that the drops of raine have their figures almost Spherical; he hath shewne from thence the cause of the colours in the Rainbow; which therefore need not be repeated.
15 Whiteness is Light; but Light perturbed by the reflexions of many beams of Light comming to the Eye together within a little space. For if Glass, or any other Diaphanous Body be reduced to very small parts by contusion or concussion; every one of those parts (if the Beams of a lucid Body be from any one point of the same reflected to the Eye) will represent to the beholder an Idea or Image of the whole lucid Body, that is to say, a Phantasme of White. For the strongest Light is the most White; and therefore many such parts will make many such Images. Wherefore if those parts lie thick and close together, those many Images will appear confusedly, and will by reason of the confused Light represent a White Colour. So that from hence may be deduced a possible cause, why Glass beaten, that is, reduced to powder, looks White. Also why Water and Snow are White; they being nothing but a heap of very small Diaphanous Bodies, namely, of little Bubbles, from whose several convex Superficies, there are by reflexion made several confused Phantasmes of the whole lucid Body; that is to say, Whiteness. For the same reason, Salt and Nitre are White; as consisting of small Bubbles which contain within them Water and Aire; as is manifest in Nitre, from this, that being thrown into the fire, it violently blowes the same; which Salt also doth, but with less violence. But if a White Body be exposed, not to the Light of the Day, but to that of the Fire, or of a Candle, it will not at the first sight be easily judged whether it be White or Yellow; the cause whereof may be this, that the light of those things which burn and flame, is almost of a middle Colour between Whiteness and Yellowness.
16 As Whiteness is Light, so Blackness is the privation of Light, or Darkness. And from hence it is; First, that all Holes, from which no light can be reflected to the Eie, appear Black. Secondly, that when a Body hath little eminent particles erected straight up from the Superficies (so that the Beams of Light which fall upon them are reflected, not to the Eie, but to the Body it self) that Superficies appears Black, in the same manner as the Sea appears Black, when ruffled by the Wind. Thirdly, that any combustible matter is by the fire made to look Black before it shines. For the endeavour of the fire being to dissipate the smallest parts of such Bodies as are thrown into it, it must first raise and erect those parts, before it can work their dissipation. If therefore the fire be put out before the parts be totally dissipated, the Cole will appear Black; for the parts being onely erected, the Beams of Light falling upon them will not be reflected to the Eie, but to the Cole it self. Fourthly, that Burning Glasses do more easily burn Black things then White. For in a White Superficies, the eminent parts are convex, like little bubbles; and therefore the Beams of Light which fall upon them are reflected every way from the reflecting Body. But in a Black Superficies, where the eminent particles are more erected, the Beams of Light falling upon them, are all necessarily reflected towards the Body it self; and therefore Bodies that are Black are more easily set on fire by the Sun-beams, then those that are White. Fifthly, that all Colours that are made of the mixture of White and Black, proceed from the different position of the particles that rise above the Superficies, and their different forms of asperity. For according to these differences, more or fewer Beams of Light are reflected from several Bodies to the Eie. But in regard those differences are innumerable, and the Bodies themselves so small, that we cannot perceive them, the explication and precise determination of the Causes of all Colours is a thing of so great difficulty, that I dare not undertake it.
CHAP. XXVIII. OF COLD, WIND, HARD, ICE, RESTITUTION OF BODIES BENT, DIAPHANOUS, LIGHTNING AND THUNDER; AND OF THE HEADS OF RIVERS.
1 WHY BREATH from the same mouth sometimes heats, and sometimes cools.
2 Wind, and the Inconstancy of Winds, whence.
3 Why there is a constant, though not a great Wind from East to West neer the Equator.
4 What is the effect of Aire pent in between the Clouds.
5 No change from Soft to Hard, but by motion.
6 What is the cause of Cold neer the Poles.
7 The cause of Ice; and why the Cold is more remiss in rainy then in clear weather. Why water doth not freeze in deep Wells, as it doth neer the Superficies of the Earth. Why Ice is not so heavy as Water; and why Wine is not so easily frozen as Water.
8 Another cause of Hardness from the fuller contact of Atomes. Also how Hard things are broken.
9 A third cause of Hardness from Heat.
10 A fourth cause of Hardness from the motion of Atomes enclosed in a narrow space.
11 How Hard things are Softned.
12 Whence proceeds the spontaneous Restitution of things Bent.
13 Diaphanous, and Opacous, what they are, and whence.
14 The cause of Lightning and Thunder.
15 Whence it proceeds that Clouds can fall again, after they are once elevated and frozen.
16 How it could be that the Moon was eclipsed, when she was not diametrally opposite to the Sunne.
17 By what means many Sunnes may appear at once.
18 Of the Heads of Rivers.
1 AS, when the motion of the ambient aethereal substance makes the Spirits and fluid parts of our Bodies tend outwards, we acknowledge Heat; so, by the endeavour inwards of the same spirits and humours, we feel Cold. So that to Cool, is to make the exterior parts of the Body endeavour inwards, by a motion contrary to that of Calefaction, by which the internal parts are called outwards. He therefore that would know the cause of Cold must find by what motion or motions, the exterior parts of any Body endeavour to retire inwards. To begin with those Phaenomena which are the most familiar; There is almost no man but knows, that breath blown strongly, and which comes from the mouth with violence, that is to say, the passage being straight, will Cool the hand; and that the same breath blown gently, that is to say, through a greater aperture, wil warm the same The cause of which Phaenomenon may be this. The breath going out, hath two motio¯s; the one, of the whole and direct, by which the formost parts of the hand are driven inwards, the other, simple motion of the small particles of the same breath, which (as I have shewn in the 3d Article of the last Chapter) causeth Heat. According therefore as either of these Motions is predominant, so there is the sense sometimes of Cold, sometimes of Heat. Wherefore, when the breath is softly breathed out at a large passage, that simple Motion which causeth Heat prevaileth, and consequently Heat is felt; and when by compressing the lips the breath is more strongly blown out, then is the direct motion prevalent, which makes us feel Cold. For the direct motion of the breath or aire, is Wind; and all Wind Cools, or diminisheth former heat.
2 And seeing not onely great Wind, but almost any Ventilation and stirring of the Aire, doth refrigerate; the reason of many experiments concerning Cold cannot well be given, without finding first what are the causes of Wind. Now Wind is nothing else but the direct motion of the Aire thrust forwards; which nevertheless, when many Winds concurre may be circular, or otherwise indirect, as it is in Whirle-winds. Wherefore in the first place we are to enquire into the Causes of Winds. Wind is Aire moved in a considerable quantity, and that either in the manner of Waves, which is both forwards & also up & down; or else forwards onely.
Supposing therefore the Aire both cleer and calm, for any time how little soever; yet the greater Bodies of the World, being so disposed and ordered as has been said, it will be necessary that a Wind presently arise some where. For seeing that motion of the parts of the Aire which is made by the Simple Motion of the Sunne in his own Epicycle, causeth an exhalation of the particles of water from the Seas and all other moist Bodies, and those particles make Clouds; it must needs follow, that whilest the particles of water pass upwards, the particles of Aire (for the keeping of all Spaces full) be justled out on every side, and urge the next particles, and these the next, till having made their circuit, there comes continually so much Aire to the hinder parts of the Earth, as there went water from before it. Wherefore the ascending Vapours move the Aire towards the sides every way; and all direct motion of the Aire being Wind, they make a Wind. And if this Wind meet often with other Vapours which arise in other places, it is manifest that the force thereof will be augmented, & the way or course of it changed. Besides, according as the Earth by its diurnal motion turns sometimes the drier, sometimes the moister part towards the Sunne, so sometimes a greater, sometimes a less quantity of Vapours will be raised, that is to say, sometimes there will be a less, sometimes a greater Wind. Wherefore I have rendred a possible cause of such Winds, as are generated by Vapours; and also of their Inconstancy.
From hence it follows, that these Winds cannot be made in any place which is higher then that to which Vapours may ascend. Nor is that incredible which is reported of the highest Mountains, as the Pique of Tenariffe and the Andes of Peru, namely, that they are not at all troubled with these inconstant Winds. And if it were certain, that neither Rain nor Snow were ever seen in the highest tops of those Mountains, it could not be doubted but that they are higher then any place to which Vapours use to ascend.
3 Nevertheless, there may be Wind there, though not that which is made by the ascent of Vapours, yet a less & more constant Wind (like the continued blast of a pair of bellows) blowing from the East. And this may have a double cause; the one, the diurnal mo tion of the Earth; the other, its simple motion in its own Epicycle. For these Mountains being (by reason of their height) more eminent then all the rest of the parts of the Earth, do by both these Motions drive the Aire from the West Eastwards. To which though the diurnal Motion contribute but little; yet seeing I have supposed that the simple Motion of the Earth in its own Epicycle, makes two revolutions in the same time in which the diurnal Motion makes but one; and that the Semidiameter of the Epicycle is double to the Semidiameter of the diurnal Conversion, the Motion of every point of the Earth in its own Epicycle will have its velocity quadruple to that of the diurnal Motion; so that by both these Motions together, the tops of those Hils will sensibly be moved against the Aire; and consequently a Wind will be felt. For whether the Air strike the Sentient, or the Sentient the Air, the perception of Motion will be the same. But this Wind, seeing it is not caused by the ascent of Vapours, must necessarily be very Constant.
4 When one Cloud is already ascended into the Aire, if another Cloud ascend towards it, that part of the Aire which is intercepted between them both, must of necessity be pressed out every way. Also when both of them, whilest the one ascends, and the other either stayes, or descends, come to be joyned in such manner as that the aethereal substance be shut within them on every side, it will by this compression also go out by penetrating the Water. But in the mean time, the hard particles which are mingled with the Aire, and are agitated (as I have supposed) with Simple Motion, wil not pass through the water of the clouds, but be more straightly compressed within their cavities. And this I have demonstrated at the 4th and 5th Articles of the 22th Chapter. Besides, seeing the Globe of the Earth floateth in the Aire which is agitated by the Sunnes Motion, the parts of the Aire resisted by the Earth, will spread themselves every way upon the Earths Superficies; as I have shewn at the 8th Article of the 21th Chapter.
5 We perceive a Body to be Hard, from this, that when touching it we would thrust forwards that part of the same which we touch, we cannot do it otherwise then by thrusting forwards the whole Body. We may indeed easily and sensibly thrust forwards any particle of the Aire or Water which we touch, whilst yet the rest of its parts remain (to sense) unmoved. But we cannot do so to any part of a stone. Wherfore I define a Hard Body to be that, whereof no part can be sensibly moved, unless the whole be moved. Whatsoever therefore is Soft or Fluid, the same can never be made Hard but by such motion, as makes many of the parts together stop the motion of some one part, by resisting the same.
6 These things premised, I shall shew a possible cause why there is greater Cold neer the Poles of the Earth, then further from them. The motion of the Sunne between the Tropicks, driving the Aire towards that part of the Earths Superficies which is perpendicularly under it, makes it spread it self every way; and the velocity of this expansion of the Aire grows greater and greater, as the Superficies of the Earth comes to be more and more straightned, that is to say, as the Circles which are parallel to the Aequator come to be less and less. Wherefore this expansive motion of the Aire, drives before it the parts of the Aire which are in its way continually towards the Poles more and more strongly, as its force comes to be more and more united, that is to say, as the Circles which are parallel to the Aequator are less and less; that is, so much the more, by how much they are neerer to the Poles of the Earth. In those places therefore which are neerer to the Poles, there is greater Cold, then in those which are more remote from them. Now this expansion of the Aire upon the Superficies of the Earth from East to West, doth by reason of the Sunnes perpetual accession to the places which are successively under it, make it Cold at the time of the Sunnes Rising and Setting; but as the Sunne comes to be continually more and more perpendicular to those cooled places, so by the Heat which is generated by the supervening Simple Motion of the Sunn, that Cold is again remitted; and can never be great, because the action by the which it was generated, is not permanent. Wherefore I have rendred a possible cause of Cold in those places that are neer the Poles, or where the obliquity of the Sunne is great.
7 How Water may be congeled by Cold, may be explained in this manner. Let A (in the first figure) represent the Sunne, and B the Earth. A will therefore be much greater then B. Let EF be in the plain of the Equinoctial; to which let GH, IK and LC be parallel. Lastly, let C and D be the Poles of the Earth. The Aire therefore by its action in those parallels will rake the Superficies of the Earth; and that with motion so much the stronger, by how much the parallel Circles towards the Poles grow less and less. From whence must arise a Wind, which will force together the uppermost parts of the water, and withal raise them a little, weakning their endeavour towards the Center of the Earth. And from their endeavour towards the center of the Earth, joyned with the endeavour of the said Wind, the uppermost parts of the water will be pressed together, and coagulated, that is to say, the top of the water will be skinned over and hardned. And so againe the Water next the top will be hardned in the same manner, till at length the Ice be thick. And this Ice being now compacted of little hard Bodies, must also containe many particles of ayre received into it.
As Rivers and Seas, so also in the same manner may the Clouds be frozen. For when by the ascending and descending of severall Clouds at the same time, the Air intercepted between them is by compression forced out, it rakes, & by little & little hardens them. And though those smal drops (which usually make Clouds) be not yet united into greater Bodies, yet the same Wind will be made; & by it, as water is congeled into Ice, so will Vapours in the same manner be congeled into Snow. From the same cause it is that Ice may be made by art, and that not farre from the fire. For it is done by the mingling of Snow and Salt together, and by burying in it a small vessell full of Water. Now while the Snow and Salt (which have in them a great deale of aire) are melting, the aire which is pressed out every way in Wind, rakes the sides of the Vessel; and as the Wind by its motion rakes the Vessell, so the Vessell by the same motion and action congeles the Water within it.
We find by experience, that Cold is allwayes more Remisse in places where it raynes, or where the weather is cloudy (things being alike in all other respects) then where the aire is cleare. And this agreeth very well with what I have sayd before. For in cleare weather, the course of the Wind which (as I sayd even now) rakes the Superficies of the Earth, as it is free from all interruption, so also it is very strong. But when small drops of water are either rising or falling, that Wind is repelled, broken and dissipated by them; and the lesse the Wind is, the lesse is the Cold.
We find also by experience, that in deep Wells the Water freezeth not so much, as it doth upon the Superficies of the Earth. For the Wind by which Ice is made, entring into the Earth (by reason of the laxity of its parts) more or lesse, loseth some of its force, though not much. So that if the Well be not deep, it will freeze; whereas if it be so deep, as that the Wind which causeth cold cannot reach it, it will not freeze.
We find moreover by experience, that Ice is lighter then Water. The cause whereof is manifest from that which I have already shewn, namely, that Aire is received in and mingled with the particles of the Water whilest it is in congeling.
8 We have seen one way of making things Hard, namely, by Congelation. Another way is thus. Having already supposed, that innumerable Atomes, some harder then others, and that have several simple motions of their own, are intermingled with the aethereal substance; it follows necessarily from hence, that by reason of the fermentation of the whole Aire (of which I have spoken in the 21 Chap.) some of those Atomes meeting with others, will cleave together, by applying themselvs to one another in such manner as is agreeable to their motions and mutual contacts; and (seeing there is no Vacuum) cannot be pulled asunder, but by so much force as is sufficient to overcome their Hardness.
Now there are innumerable degrees of Hardness. As (for example) there is a degree of it in Water, as is manifest from this, that upon a plain it may be drawn any way at pleasure by ones finger. There is a greater degree of it in clammy liquors; which when they are poured out, doe in falling downwards dispose themselves into one continued thred; which thred before it be broken will by little and little diminish its thickness, till at last it be so small, as that it seems to break onely in a point; and in their separation the external parts break first from one another, and then the more internal parts successively one after another. In Wax there is yet a greater degree of Hardness. For when we would pull one part of it from another, we first make the whole mass slenderer, before we can pull it asunder. And how much the harder anything is which we would break, so much the more force we must apply to it. Wherefore, if we go on to harder things, as Ropes, Wood, Metals, Stones, &c. reason prompteth us to believe that the same (though not alwayes sensibly) will necessarily happen; and that even the hardest things are broken asunder in the same manner, namely, by Solution of their continuity, begun in the outermost Superficies, and proceeding successively to the innermost parts. In like manner, when the parts of Bodies are to be separated, not by pulling them asunder, but by breaking them, the first separation will necessarily be in the convex Superficies of the bowed part of the Body, and afterwards in the concave Superficies. For in all bowing, there is in the convex Superficies an endeavour in the parts to go one from another, and in the concave Superficies to penetrate one another.
This being well understood, a reason may be given, how two Bodies which are contiguous in one common Superficies, may by force be separated without the introduction of Vacuum; though Lucretius thought otherwise, believing that such separation was a strong establishment of Vacuum. For a Marble Pillar being made to hang by one of its bases, if it be long enough it will by its own weight be broken asunder, and yet it will not necessarily follow that there should be Vacuum, seeing the solution of its continuity may begin in the Circumference, and proceed successively to the midst thereof.
Lastly, Wine is not so easily congeled as Water, because in Wine there are particles which being not fluid, are moved very swiftly, and by their motion congelation is retarded; but if the Cold prevail against this motion, then the outermost parts of the Wine will be first frozen, and afterwards the inner parts; whereof this is a signe, that the Wine which remains unfrozen in the midst will be very strong.
9 Another cause of Hardness in some things may be in this manner. If a soft Body consist of many hard particles, which by the intermixture of many other fluid particles cohaere but loosely together, those fluid parts (as hath been shewn in the last Article of the 21 Chapter) will be exhaled; by which means each hard particle will apply it self to the next to it according to a greater Superficies; and consequently they will cohaere more closely to one another; that is to say, the whole mass will be made Harder.
10 Again, in some things Hardness may be made to a certain degree, in this manner. When any fluid substance hath in it certain very small Bodies intermingled, which being moved with simple motion of their own, contribute like motion to the parts of the fluid substance, and this be done in a small enclosed space (as in the hollow of a little Sphere, or a very slender Pipe) if the motion be vehement, and there be a great number of these small enclosed Bodies, two things will happen; the one, that the fluid substance will have an endeavour of dilating it self at once every way; the other, that if those smal Bodies can no where get out, then from their reflexion it will follow, that the motion of the parts of the enclosed fluid substance, which was vehement before, will now be much more vehement. Wherefore if any one particle of that fluid substance should be touched & pressed by some external Movent, it could not yeild but by the application of very sensible force. Wherefore the fluid substance which is enclosed, and so moved, hath some degree of Hardness. Now greater and less degree of Hardness depends upon the quantity and velocity of those small Bodies and upon the narrowness of the place both together.
11 Such things as are made Hard by sudden heat, namely, such as are hardned by fire, are commonly reduced to their former soft form by Maceration. For fire hardens by Evaporation, and therefore if the evaporated moisture be restored again, the former nature and form is restored together with it. And such things as are frozen with Cold, if the Wind by which they were frozen change into the opposite quarter, they will be unfrozen again (unless they have gotten a habit of new motion or endeavour by long continuance in that hardness). Nor is it enough to cause thawing, that there be a cessation of the freezing Wind (for the taking away of the Cause doth not destroy a produced effect); but the thawing also must have its proper cause, namely, a contrary Wind, or at least a Wind opposite in some degree. And this we finde to be true by experience. For if Ice be laid in a place so well enclosed that the motion of the Aire cannot get to it, that Ice will remain unchanged, though the place be not sensibly cold.
12 Of Hard Bodies, some may manifestly be bowed; others not, but are broken in the very first moment of their bending. And of such Bodies as may manifestly be bended, some being bent, do as soon as ever they are set at liberty, Restore themselves to their former posture, others remain still bent. Now if the cause of this Restitution be asked, I say it may be in this manner; namely, that the particles of the bended Body, whilest it is held bent, do nevertheless retain their motion; and by this motion they restore it as soon as the force is removed by which it was bent. For when any thing is bent (as a plate of steel), and as soon as the force is removed restores it self again, it is evident that the cause of its restitution cannot be referred to the ambient aire; nor can it be referred to the removal of the force by which it was bent; for in things that are at rest, the taking away of impediments, is not a sufficient cause of their future Motion; there being no other cause of Motion, but Motion. The cause therefore of such Restitution is in the parts of the Steel it self. Wherefore whilest it remains bent, there is in the parts of which it consisteth, some motion, though invisible, that is to say, some endeavour at least that way by which the restitution is to be made; and therefore this endeavour of all the parts together is the first beginning of Restitution; so that the impediment being removed, that is to say, the force by which it was held bent, it will be restored again. Now the motion of the parts by which this is done, is that which I called Simple Motion, or Motion returning into it self. When therefore in the bending of a plate, the ends are drawn together, there is on one side a mutual compression of the parts; which compression is one endeavour opposite to another endeavour; and on the other side a divulsion of the parts. The endeavour therefore of the parts on one side tends to the restitution of the plate from the middle towards the ends; and on the other side, from the ends towards the middle. Wherefore the impediment being taken away, this endeavour (which is the beginning of restitution) will restore the plate to its former posture. And thus I have given a possible cause why some Bodies when they are bent Restore themselves again; which was to be done.
As for Stones, seeing they are made by the accretion of many very hard particles within the Earth; which particles have no great coherence, that is to say, touch one another in small latitude, and consequently admit many particles of aire, it must needs be that in bending of them, their internal parts will not easily be compressed by reason of their hardness. And because their coherence is not firm, as soon as the external hard particles are disjoyned, the aethereal parts will necessarily break out, and so the Body will suddenly be broken.
13 Those Bodies are called Diaphanous, upon which whilest the Beams of a lucid Body do work, the action of every one of those Beams is propagated in them in such manner as that they still retain the same order amongst themselves, or the inversion of that order; and therefore Bodies which are perfectly Diaphanous, are also perfectly homogeneous. On the contrary, an Opacous Body is that, which by reason of its heterogeneous nature, doth by innumerable reflexions and refractions in particles of different figures and unequal hardness, weaken the Beams that fall upon it before they reach the Eie. And of Diaphanous Bodies, some are made such by Nature from the beginning; as the substance of the Aire, and of the Water, and perhaps also some parts of Stones, unless these also be Water that has been long congeled. Others are made so by the power of Heat, which congregates homogeneons Bodies. But such as are made Diaphanous in this manner, consist of parts which were formerly Diaphanous.
14 In what manner Clouds are made by the motion of the Sunne, elevating the particles of Water from the Sea and other moist places, hath been explained in the 26th Chapter. Also how Clouds come to be frozen, hath been shewn above at the 7th Article. Now from this, that Aire may be enclosed, as it were in Caverns, and pent together more and more by the meeting of ascending and descending Clouds, may be deduced a possible Cause of Thunder and Lightening. For seeing the Aire consists of two parts, the one Aethereal, which has no proper motion of its own, as being a thing divisible into the least parts; the other Hard, namely, consisting of many hard Atomes which have every one of them a very swift simple motion of its own; whilest the Clouds by their meeting do more and more straighten such Cavities as they intercept, the Aethereal parts will penetrate and pass through their watry substance; but the hard parts will in the mean time be the more thrust together, and press one another; and consequently (by reason of their vehement motions) they will have an endeavour to rebound from each other. Whensoever therefore the compression is great enough, and the concave parts of the Clouds are (for the cause I have already given) congeled into Ice, the Cloud wil necessarily be broken; & this breaking of the Cloud produceth the first clap of Thunder. Afterwards, the Aire which was pent in, having now broken through, makes a concussion of the Aire without; and from hence proceeds the roaring and murmur which follows; and both the first Clap and the Murmur that follows it, make that noise which is called Thunder. Also from the same Aire breaking through the Clouds, and with concussion falling upon the Eie, proceeds that action upon our Eie, which causeth in us a perception of that Light which we call Lightening. Wherefore I have given a possible cause of Thunder and Lightening.
15 But if the Vapours which are raised into Clouds, do run together again into Water, or be congeled into Ice, from whence is it (seeing both Ice and Water are heavy) that they are sustained in the Aire? Or rather, what may the cause be, that being once elevated, they fall down again? For there is no doubt but the same force which could carry up that Water, could also sustain it there. Why therefore being once carried up, doth it fall again? I say it proceeds from the same Simple Motion of the Sunne, both that Vapours are forced to ascend, and that Water gathered into Clouds is forced to descend. For in the 21th Chapter and 11th Article I have shewn how Vapours are elevated; and in the same Chapter and 5th Article I have also shewn how by the same motion Homogeneous Bodies are congregated, & Heterogeneous dissipated; that is to say, how such things as have a like nature to that of the Earth, are driven towards the Earth; that is to say, what is the cause of the descent of Heavy Bodies. Now if the action of the Sun be hindered in the raising of vapours, and be not at all hindered in the casting of them down, the Water will descend. But a Cloud cannot hinder the action of the Sunne in making things of an earthly nature descend to the Earth, though it may hinder it in making Vapours ascend For the lower part of a thick Cloud is so covered by its upper part, as that it cannot receive that action of the Sunne by which Vapours are carried up, because Vapours are raised by the perpetual fermentation of the Aire, or by the separating of its smallest parts from one another, which is much weaker when a thick Cloud is interposed, then when the Skie is cleere. And therefore whensoever a Cloud is made thick enough, the water which would not descend before, will then descend, unless it be kept up by the agitation of the Winde. Wherefore I have rendred a possible cause, both why the Clouds may be sustained in the Aire, and also why they may fall down again to the Earth; which was propounded to be done.
16 Granting that the Clouds may be frozen, it is no wonder if the Moon have been seen eclipsed at such time as she hath been almost two degrees above the Horizon, the Sunne at the same time appearing in the Horizon; for such an Eclipse was observed by Mestline at Tubing in the year 1590. For it might happen that a frozen Cloud was then interposed between the Sunne and the Eie of the Observer. And if it were so, the Sunne which was then almost two Degrees below the Horizon, might appear to be in it, by reason of the passing of his Beams through the Ice. And it is to be noted, that those that attribute such refractions to the Atmosphere, cannot attribute to it so great a refraction as this. Wherefore not the Atmosphere, but either Water in a continued Body, or else Ice must be the cause of that refraction.
17 Again, granting that there may be Ice in the Clouds, it will be no longer a wonder that many Sunnes have sometimes appeared at once. For Looking-glasses may be so placed, as by reflections to shew the same object in many places. And may not so many frozen Clouds serve for so many Looking-glasses? and may they not be fitly disposed for that purpose? Besides, the number of Appearances may be encreased by refractions also; and therefore it would be a greater wonder to me, if such Phaenomena as these should never happen.
And were it not for that one Phaenomenon of the new Starre which was seen in Cassiopaea, I should think Comets were made in the same manner, namely, by Vapours drawn not onely from the Earth, but from the rest of the Planets also, and congeled into one continued Body. For I could very well from hence give a reason both of their Haire, and of their motions. But seeing that Starre remained sixteen whole moneths in the same place amongst the fixed Starres, I cannot believe the matter of it was Ice. Wherefore I leave to others, the disquisition of the cause of Comets; concerning which, nothing that hath hitherto been published, ( the bare Histories of them) is worth considering.
18 The Heads of Rivers may be deduced from Rain-water, or from melted Snowes, very easily; but from other causes, very hardly, or not at all. For both Rain-water, and melted Snowes run down the descents of Mountains; and if they descend onely by the outward Superficies, the Showres or Snowes themselves may be accounted the Springs or Fountains; but if they enter the Earth & descend within it, then wheresoever they break out, there are their Springs. And as these Spings make small streams, so, many small streams running together make Rivers. Now there was never any Spring fou¯d, but where the Water wch flowed to it, was either further, or at least as farre from the center of the Earth, as the Spring it self. And whereas it has bin objected by a great Philosopher, that in the top of Mount-Cenis (which parts Savoy from Piemont) there Springs a River which runs down by Susa; it is not true. For there are above that River, for two miles length, very high hils on both sides, which are almost perpetually covered with Snow; from which, innumerable little streams running down do manifestly supply that River with water sufficient for its magnitude.
CHAP. XXIX. OF SOUND, ODOUR, SAVOUR, AND TOUCH
1 THE DEFINITION of Sound, and the distinctions of Sounds.
2 The cause of the degrees of Sounds.
3 The difference between Sounds Acute and Grave.
4 The difference between Clear and Hoarse Sounds, whence.
5 The Sound of Thunder and of a Gunne, whence it proceeds.
6 Whence it is, that Pipes by blowing into them have a clear Sound.
7 Of Reflected Sound.
8 From whence it is that Sound is Uniform and Lasting.
9 How Sound may be helped aud hindered by the Wind.
10 Not onely Aire, but other Bodies how hard soever they be, conveigh Sound.
11 The causes of Grave and Acute Sounds, and of Concent.
12 Phaenomena for Smelling.
13 The first Organ and the generation of Smelling.
14 How it is helped by Heat and by Wind.
15 Why such Bodies are least smelt, which have least intermixture of Aire in them.
16 Why Odorous things become more Odorous by being bruised.
17 The first Organ of Tasting; and why some Savours cause Nauseousness.
18 The first Organ of Feeling; and how we come to the knowledge of such Objects as are common to the Touch and other Senses.
SOUND is Sense generated by the action of the Medium, when its motion reacheth the Eare and the rest of the Organs of Sense. Now the motion of the Medium is not the Sound it self, but the cause of it. For the Phantasme which is made in us, that is to say, the Reaction of the Organ is properly that which we call Sound.
The principal distinctions of Sounds are these; First, that one Sound is stronger, another Weaker. Secondly, that one is more Grave, another more Acute. Thirdly, that one is Clear, another Hoarse. Fourthly, that one is Primary, another Derivative. Fifthly, that one is Uniform, another not. Sixthly, that one is more Durable, another less Durable. Of all which distinctions the members may be subdistinguished into parts distinguishable almost infinitely. For the variety of Sounds seems to be not much less then that of Colours.
As Vision, so Hearing is generated by the motion of the Medium, but not in the same manner. For Sight is from Pressure, that is, from an Endeavour; in which there is no perceptible progression of any of the parts of the Medium; but one part urging or thrusting on an other, propagateth that action successively to any distance whatsoever; whereas the motion of the Medium by which Sound is made, is a Stroke. For when we Hear, the Drumme of the Eare (which is the first Organ of Hearing) is stricken; and the Drumme being stricken, the Pia Mater is also shaken, and with it the Arteries which are inserted into it; by which the action being propagated to the Heart it self, by the reaction of the Heart a Phantasm is made which we call Sound; and (because the reaction tendeth outwards) we think it is without.
2 And seeing the effects produced by Motion, are greater or lesse, not onely when the Velocity is greater or less, but also when the Body hath greater or less Magnitude though the Velocity be the same; a Sound may be greater or lesse both these wayes. And because neither the greatest nor the least Magnitude or Velocity can be given, it may happen, that either the motion may be of so small velocity, or the Body it self of so small magnitude, as to produce no Sound at all; or either of them may be so great, as to take away the Faculty of Sense by hurting the Organ.
From hence may be deduced possible causes of the strength and weakness of Sounds in the following Phaenomena.
The first whereof is this, That if a man speak through a Trunk which hath on end applyed to the mouth of the Speaker, and the other to the eare of the Hearer, the Sound will come stronger then it would do through the open Aire. And the cause (not onley the possible, but the certain and manifest cause) is this, that the Aire which is moved by the first breath, and carried forwards in the Trunk, is not diffused, as it would be in the open Aire, and is consequently brought to the eare almost with the same velocity with which it was first breathed out; Whereas in the open Aire, the first motion diffuseth it self every way into Circles, such as are made by the throwing of a Stone into a standing water, where the velocity grows less and less as the Undulation proceeds further and further from the beginning of its motion.
The second is this, That if the Trunk be short, and the end which is applyed to the mouth be wider then that which is applyed to the eare, thus also the Sound will be stronger then if it were made in the open aire. And the cause is the same, namely, that by how much the wider end of the Trunk is less distant from the beginning of the Sound, by so much the less is the diffusion.
The third, That it is easier for one that is within a Chamber, to heare what is spoken without, then for him that stands without, to hear what is spoken within. For the Windows and other inlets of the moved Aire, are as the wide end of the Trunk. And for this reason some creatures seem to hear the better, because Nature has bestowed upon them wide and capacious Ears.
The fourth is this, That though he which standeth upon the Sea shore, cannot heare the Collision of the two neerest waves, yet neverthess he hears the roaring of the whole Sea. And the cause seems to be this, that though the several collisions move the Organ, yet they are not severally great enough to cause Sense; whereas nothing hinders but that all of them together may make Sound.
3 That Bodies when they are stricken do yeild some a more Grave, others a more Acute Sound, the cause may consist in the difference of the times in which the parts stricken and forced out of their places, return to the same places again. For in some Bodies, the restitution of the moved parts is quick, in others slow. And this also may be the cause why the parts of the Organ which are moved by the Medium, return to their rest again, sometimes sooner, sometimes later. Now by how much the Vibrations, or the reciprocal motions of the parts are more frequent, by so much doth the whole Sound made (at the same time) by one stroke, consist of more, and consequently of smaller parts. For what is Acute in Sound, the same is Subtle in Matter; and both of them, namely, Acute Sound, and Subtle Matter consist of very small parts, that of Time, and this of the Matter it self.
The third distinction of Sounds cannot be conceived clearly enough by the names I have used of Clear and Hoarse, nor by any other that I know; and therefore it is needful to explain them by examples. When I say Hoarse, I understand Whispering and Hissing, and whatsoever is like to these, by what appellation soever it be expressed. And Sounds of this kind seem to be made by the force of some strong Wind, raking rather then striking such hard Bodies, as it falls upon. On the contrary, when I use the word Clear, I do not understand such a Sound as may be easily and distinctly heard, for so Whispers would be Clear, but such as is made by somewhat that is Broken; and such as is Clamor, Tinkling, the Sound of a Trumpet, &c. and (to express it significantly in one word), Noise. And seeing no Sound is made but by the concourse of two Bodies at the least, by which concourse it is necessary that there be as well Reaction as Action, that is to say, one motion opposite to another; it follows, that according as the proportion between those two opposite motions is diversified, so the Sounds which are made will be different from one another. And whensoever the proportion between them is so great, as that the motion of one of the Bodies be insensible if compared with the motion of the other, then the Sound will not be of the same kind; as when the Wind falls very obliquely upon a hard Body, or when a hard Body is carried swiftly through the Aire; for then there is made that Sound which I call a Hoarse Sound, in Greek. Therefore the breath blown with violence from the mouth makes a Hissing, because in going out it rakes the Superficies of the Lips, whose reaction against the force of the breath is not Sensible. And this is the cause why the Winds have that Hoarse Sound. Also if two Bodies how hard soever, be rubbed together with no great pressure, they make a Hoarse Sound. And this Hoarse Sound when it is made (as I have said) by the Aire raking the Superficies of a hard Body, seemeth to be nothing but the dividing of the Aire into innumerable and very small Files. For the asperity of the Superficies doth by the eminencies of its innumerable parts divide or cut in pieces the Aire that slides upon it.
4 Noise, or that which I call Clear Sound, is made two wayes; one, by two Hoarse Sounds, made by opposite motions; the other, by Collision, or by the suddain pulling asunder of two Bodies, whereby their small particles are put into commotion, or being already in commotion, suddenly restore themselves again; which motion making impression upon the Medium, is propagated to the Organ of Hearing. And seeing there is in this Collision, or divulsion, an endeavour in the particles of one Body, opposite to the endeavour of the particles of the other Body, there will also be made in the Organ of Hearing a like opposition of endeavours, that is to say, of motions; and consequently the Sound arising from thence, will be made by two opposite motions, that is to say, by two opposite Hoarse Sounds in one and the same part of the Organ. For (as I have already said) a Hoarse Sound supposeth the sensible motion of but one of the Bodies. And this opposition of motions in the Organ is the cause why two Bodies make a Noyse, when they are either suddenly stricken against one another, or suddenly broken asunder.
5 This being granted; and seeing withall, that Thunder is made by the vehement eruption of the Aire out of the cavities of congeled Clouds; the cause of the great Noyse or Clap, may be the suddain breaking asunder of the Ice. For in this action it is necessary, that there be not onely a great concussion of the small particles of the broken parts, but also that this Concussion (by being communicated to the Aire) be carried to the Organ of Hearing, & make impression upon it. And then, from the first reaction of the Organ proceeds that first and greatest Sound, which is made by the collision of the parts whilst they restore themselves. And seeing there is in all Concussion a reciprocation of Motion forwards and backwards in the parts stricken, (for opposite motions cannot extinguish one another in an instant, as I have shewn in the 11th Art. of the 8th Chap.) it follows necessarily, that the Sound will both continue, and grow weaker and weaker, till at last the action of the reciprocating aire grow so weak, as to be unperceptible. Wherefore a possible cause is given both of the first fierce Noyse of the Thunder; and also of the Murmur that follows it.
The cause of the great Sound from a discharged piece of Ordnance, is like that of a Clap of Thunder. For the Gunpowder being fired, doth in its endeavour to go out, attempt every way the sides of the metal in such manner, as that it enlargeth the Circumference all along, and withall shortneth the axis; so that whilest the peece of Ordnance is in discharging, it is made both wider and shorter then it was before; and therefore also presently after it is discharged its wideness will be diminished, and its length encreased again by the restitution of all the particles of the matter of which it consisteth to their former position. And this is done with such motion of the parts, as are not onely very vehement, but also opposite to one another; which motions being communicated to the Aire, make impression upon the Organ, and by the reaction of the Organ create a Sound; which lasteth for some time, as I have already shewn in this Article.
I note by the way (as not belonging to this place) that the possible cause why a Gun recoyles when it is shot off, may be this; That being first swoln by the force of the fire, and afterwards restoring it self; from this restitution there proceeds an endeavour from all the sides towards the cavity; and consequently this endeavour is in those parts which are next the breech; which being not hollow, but solid, the effect of the restitution is by it hindered and diverted into the length; and by this means both the breech and the whole Gun is thrust backwards; and the more forcibly by how much the force is greater by which the part next the breech is restored to its former posture; that is to say, by how much the thiner is that part. The cause therefore why Gunnes recoyle, some more, some less, is the difference of their thickness towards the breech; & the greater that thickness is, the less they recoyl; and contrarily.
6 Also the cause why the Sound of a Pipe, which is made by blowing into it, is nevertheless Clear, is the same with that of the Sound which is made by collision. For if the breath when it is blown into a Pipe doe onely rake its concave Superficies, or fall upon it with a very sharp angle of incidence, the Sound will not be Clear, but Hoarse. But if the angle be great enough, the percussion which is made against one of the hollow sides, will be reverberated to the opposite side; and so successive repercussions will be made from side to side, till at last the whole concave Superficies of the Pipe be put into motion; which motion will be reciprocated, as it is in Collision; and this reciprocation being propagated to the Organ, from the reaction of the Organ will arise a Cleare Sound, such as is made by Collision, or by breaking asunder of hard Bodies.
In the same manner it is with the Sound of a Mans voice. For when the breath passeth out, without interruption, and doth but lightly touch the cavities through which it is sent, the Sound it maketh is a Hoarse Sound. But if in going out it strike strongly upon the Larinx, then a Clear Sound is made, as in a Pipe. And the same breath, as it comes in divers manners to the Palate, the Tongue, the Lips, the Teeth, and other Organs of Speech, so the Sounds into which it is articulated become different from one another.
7 I call that Primary Sound, which is generated by motion from the sounding Body to the Organ in a straight line without reflexion; and I call that Reflected Sound, which is generated by one or more reflexions; being the same with that we call Echo; and is iterated as often as there are reflexions made from the Object to the Eare. And these reflexions are made by Hils, Wals, and other resisting Bodies, so placed, as that they make more or fewer reflexions of the motion, according as they are themselves more or fewer in number; and they make them more or less frequently, according as they are more or less distant from one another. Now the cause of both these things is to be sought for in the situation of the reflecting Bodies, as is usually done in Sight. For the Lawes of Reflexion are the same in both, namely, that the Angles of Incidence and Reflexion be equal to one another. If therefore in a hollow Elliptique Body whose inside is well polished, or in two right Parabolical Solids which are joyned together by one common base, there be placed a Sounding Body in one of the Burning Points, & the Ear in the other, there will be heard a Sound by many degrees greater then in the open Aire; and both this, and the burning of such combustible things, as being put in the same places are set on fire by the Sun-beams, are effects of one and the same cause. But as when the visible Object is placed in one of the Burning Points, it is not distinctly seen in the other, because every part of the Object being seen in every line which is reflected from the Concave Superficies to the Eie, makes a confusion in the Sight; so neither is Sound heard articulately and distinctly when it comes to the Eare in all those reflected lines. And this may be the reason, why in Churches which have arched rooffs, though they be neither Elliptical nor Parabolical; yet because their figure is not much different from these, the voice from the Pulpit will not be heard so articulately as it would be if there were no vaulting at all.
8 Concerning the Uniformity and Duration of Sounds, both which have one common cause, we may observe, that such Bodies as being stricken yeild an unequal or harsh Sound, are very heterogeous, that is to say, they consist of parts which are very unlike both in figure and hardness, such as are Wood, Stones, and others not a few. When these are stricken, there follows a concussion of their internal particles, and a restitution of them again. But they are neither moved alike, nor have they the same action upon one another; some of them recoyling from the stroke whilest others which have already finished their recoylings are now returning; by which means they hinder and stop on another. And from hence it is that their motions are not only unequal and harsh, but also that their reciprocations come to be quickly extinguished. Whensoever therfore this motion is propagated to the Eare, the Sound it makes is Unequal and of small Duration. On the contrary, if a Body that is stricken, be not onely sufficiently hard, but have also the particles of which it consisteth like to one another both in hardness and figure, (such as are the particles of Glass and Metals, which being first melted do afterwards settle and harden) the Sound it yeildeth, will (because the motions of its parts and their reciprocations are like and Uniform) be Uniform and pleasant, and be more or less Lasting according as the Body stricken hath gteater or less magnitude. The possible cause therefore of Sounds Uniform, and Harsh, and of their longer or shorter Duration, may be one and the same likeness and unlikeness of the internal parts of the Sounding Body, in respect both of their figure and hardness.
Besides, if two plain Bodies of the same matter, and of equal thickness, do both yeild an Uniform Sound, the Sound of that Body which hath the greatest extent of length will be the longest heard. For the motion which in both of them hath its beginning from the point of percussion, is to be propagated in the greater Body through a greater Space, and consequently that propagation will require more time; and therefore also the parts which are moved will require more time for their return. Wherefore all the reciprocations cannot be finished but in longer time; and being carried to the Eare, will make the Sound last the longer. And from hence it is manifest, that of hard Bodies which yeild an Uniform Sound, the Sound lasteth longer which comes from those that are round and hollow, then from those that are plain, if they be like in all other respects. For in circular lines the action which begins at any point, hath not fro¯ the figure any end of its propagation, because the line in which it is propagated returns again to its beginning; so that the figure hinders not but that the motion may have infinite progression; whereas in a plain, every line hath its magnitude finite, beyond which the action cannot proceed. If therefore the matter be the same, the motion of the parts of that Body whose figure is round and hollow, wil last longer, then of that which is plain.
Also, if a string which is stretched, be fastned at both ends to a hollow Body, and be stricken, the Sound will last longer then if it were not so fastned; because the trembling or reciprocation which it receives from the stroke, is by reason of the connexion communicated to the hollow Body; and this trembling, if the hollow Body be great, will last the longer by reason of that greatness. Wherefore also (for the reason above mentioned) the Sound will last the longer.
9 In Hearing, it happens (otherwise then in Seeing) that the action of the Medium is made stronger by the Wind when it blows the same way, and weaker when it blows the contrary way. The cause whereof cannot proceed from any thing but the different generation of Sound and Light. For in the generation of Light, none of the parts of the Medium between the object and the Eie are moved from their own places to other places sensibly distant; but the action is propagated in spaces imperceptible; so that no contrary Wind can diminish, nor favourable Winde encrease the Light, unless it be so strong as to remove the Object further off, or bring it nearer to the Eie. For the Wind, that is to say, the aire moved, doth not by its interposition between the object and the eie, worke otherwise then it would doe if it were stil and calme. For where the pressure is perpetuall, one part of the aire is no sooner carried away, but another by succeeding it receives the same impression which the part carried away had received before. But in the generation of Sound, the first collision or breaking asunder, beateth away & driveth out of its place the nearest part of the aire, and that to a considerable distance, and with considerable velocity; and as the circles grow (by their remotenesse) wider and wider, so the aire being more & more dissipated, hath also its motion more & more weakned. Whensoever therfore the air is so stricken as to cause Sound, if the Wind fall upon it, it will move it all, neerer to the Eare if it blow that way, and further from it if it blow the contrary way; so that according as it blowes from or towards the Object, so the Sound which is heard will seeme to come from a neerer or remoter place; and the action by reason of the unequall distances be strengthened or debilitated.
From hence may be understood the reason, why the voice of such as are said to speake in their bellies, though it be uttered neer hand, is neverthelesse heard by those that suspect nothing, as if it were a great way off. For having no former thought of any determined place from which the voice should proceed, and judging according to the greatesse, if it be weake they thinke it a great way off, if strong neer. These Ventriloqui therefore by forming their voice, not (as others) by the emission of their breath, but by drawing it inwards, doe make the same appear small and weake; which weaknesse of the voice deceives those that neither suspect the artifice, nor observe the endeavour which they use in speaking; and so instead of thinking it weake they thinke it farre off.
10 As for the Medium which conveighs Sound, it is not Aire onely. For Water, or any other Body how hard soever may be that Medium. For the Motion may be propagated perpetually in any hard continuous Body; but by reason of the difficulty with which the parts of hard Bodies are moved, the motion in going out of hard matter makes but a weak impression upon the Aire. Nevertheless if one end of a very long and hard beam be stricken, & the eare be applyed at the same time to the other end, so that when the action goeth out of the beam, the aire which it striketh may immediately be received by the eare, and be carried to the Tympanum, the Sound will be considerably strong.
In like manner, if in the night (when all other noyse, which may hinder Sound, ceaseth) a man lay his eare to the ground, he will hear the Sound of the steps of Passengers, though at a great distance; because the motion which by their treading they communicate to the earth, is propagated to the eare by the uppermost parts of the earth which receiveth it from their feet.
11 I have shewn above, that the difference between Grave and Acute Sounds consisteth in this, that by how much the shorter the time is, in which the reciprocations of the parts of a Body stricken are made, by so much the more Acute will be the Sound. Now by how much a Body of the same bigness, is either more heavy, or less stretched, by so much the longer will the reciprocations last; and therefore heavier, and less stretched Bodies (if they be like in all other respects) will yeild a Graver Sound then such as be lighter and more stretched.
As for the Concent of Sounds, it is to be considered, that the reciprocation or Vibration of the Aire (by which Sound is made) after it hath reached the Drumme of the eare, imprinteth a like Vibration upon the air that is enclosed within it; by which means the sides of the Drumme within, are stricken alternately. Now the Concent of two Sounds consists in this, that the Tympanum receive its stroke from both the Sounding Bodies in equal, & equally freque¯t spaces of time; So that when two strings make their Vibrations in the same times, the Concent they produce, is the most exquisite of all other. For the sides of the Tympanum, (that is to say, of the Organ of Hearing) will be stricken by both those Vibrations together at once, on one side or other. For example, if the two equal strings A B and C D be stricken together, and the latitudes of their Vibrations E F and G H be also equal, and the points E, G, F & H be in the concave superficies of the Tympanum, so that it receive strokes from both the strings together in E and G, and again together in F and H, the Sound which is made by the Vibrations of each string, will be so like, that it may be taken for the same Sound, and is called Unison; which is the greatest Concord. Again, the string A B retaining still its former Vibration E F, let the string C D be stretched, till its Vibration have double the swiftness it had before, and let E F be divided equally in I. In what time therefore the string C D makes one part of its Vibration from G to H, in the same time the string A B will make one part of its Vibration from E to I; and in what time the string C D hath made the other part of its Vibration back from
H to G, in the same time another part of the Vibration of the string A B will be made from I to F. But the points F and G are both in the sides of the Organ, and therefore they will strike the Organ both together, not at every stroke, but at every other storke. And this is the neerest Concord to Unison, and makes that Sound which is called an Eighth. Again, the Vibration of the string A B remayning still the same it was, let C D be stretched till its Vibration be swifter then that of the string A B, in the proportion of 3 to, and let E F be divided into three equal parts in K and L. In what time therefore the string C D makes one third part of its Vibration, which third part is from G to H, the string A B will make one third part of its Vibration, that is to say, two thirds of E F, namely, E L. And in what time the string C D makes another third part of its Vibration, namely H G, the string A B will make another third part of its Vibration, namely, from L to F, and back again from F to L. Lastly, whilest the string C D makes the last third part of its Vibration, that is, from G to H, the string A B will make the last third part of its Vibration from L to E. But the points E and H are both in the sides of the Organ. Wherefore at every third time the Organ will be stricken by the Vibration of both the strings together, and make that Concord which is called a Fifth.
12 For the finding out of the cause of Smels, I shall make use of the evidence of these following Phaenomena. First, that Smelling is hindred by Cold, and helped by Heat. Secondly, that when the Wind bloweth from the Object, the Smel is the stronger; and contrarily when it bloweth from the Sentient towards the Object, the weaker; both which Phaenomena are by experience manifestly found to be true in Doggs which follow the track of Beasts by the Sent. Thirdly, that such Bodies as are less pervious to the fluid Medium, yeild less Smell then such as are more pervious; as may be seen in Stones and Metals, which compared with Plants and Living Creatures, and their Parts, Fruits and Excretions, have very little or no Smell at all. Fourthly, that such Bodies as are of their own nature Odorous, become yet more Odorous when they are bruised. Fifthly, that when the breath is stopped (at least in Men) nothing can be Smelt. Sixthly, that the sense of Smelling is also taken away by the stopping of the Nostrils, though the mouth be left open.
13 By the fourth and fifth Phaenomenon it is manifest, that the first and immediate Organ of Smelling is the innermost cuticle of the Nostrils, and that part of it which is below the passage common to the Nostrils and the Palate. For when we draw breath by the Nostrils, we draw it into the Lungs. That breath therefore which conveighs Smels, is in the way which passeth to the Lungs, that is to say, in that part of the Nostrils which is below the passage through which the breath goeth. For nothing is Smelt, neither beyond the passage of the breath within, nor at all without the Nostrils.
And seeing that from different Smels there must necessarily proceed some mutation in the Organ, and all mutation is motion; it is therefore also necessary that in Smelling, the parts of the Organ, that is to say, of that internal cuticle, and the nerves that are inserted into it, must be diversly moved by different Smels. And seeing also that it hath been demonstrated, that nothing can be moved but by a Body that is already moved and contiguous; and that there is no other Body contiguous to the internal membrane of the nostrils, but breath, that is to say, attracted aire, and such little solid invisible Bodies (if there be any such) as are intermingled with the aire; it follows necessarily, that the cause of Smelling is either the motion of that pure aire or aethereal Substance, or the motion of those small Bodies. But this motion is an effect proceding from the Object of Smell, and therefore either the whole Object it self, or its several parts must necessarily be moved. Now we know, that Odorous Bodies make Odour though their whole bulk be not moved. Wherefore the cause of Odour is the motion of the invisible parts of the Odorous Body. And these invisible parts do either go out of the Object, or else retaining their former situation with the rest of the parts, are moved together with them, that is to say, they have simple and invisible motion. They that say there goes something out of the Odorous Body, call it an Effluvium; which Effluvium is either of the aethereal substance, or of the small Bodies, that are intermingled with it. But that all variety of Odours should proceed from the Effluviums of those small Bodies that are intermingled with the aethereal substance, is altogether incredible, for these considerations; First, that certain Unguents, though very little in quantity, do nevertheless send forth very strong Odours, not onely to a great distance of place, but also to a great continuance of time, and are to be Smelt in every point both of that place and time; so that the parts issued out are sufficient to fil ten thousand times more space, then the whole Odorous Body is able to fill; which is impossible. Secondly, that whether that issuing out be with straight or with crooked motion, if the same quantity should flow from any other Odorous Body with the same motion, it would follow, that all Odorous Bodies would yeild the same Smell. Thirdly, that seeing those Effluviums have great Velocity of motion (as is manifest from this, that noysome Odours proceeding from caverns are presently Smelt at a great distance) it would follow, that by reason there is nothing to hinder the passage of those Effluviums to the Organ, such motion alone were sufficient to cause Smelling. Which is not so; for we cannot Smell at all unless we draw in our breath through our Nostrils. Smelling therefore is not caused by the Effluvium of Atomes; nor, for the same reason is it caused by the Effluvium of aethereal substance; for so also we should Smell without the drawing in of our breath. Besides the aethereal substance being the same in all Odorous Bodies, they would always affect the Organ in the same manner, and consequently the Odours of all things would be like.
It remains therefore, that the cause of Smelling must consist in the Simple motion of the parts of Odorous Bodies, without any efflux or diminution of their whole substance. And by this motion, there is propagated to the Organ by the intermediate aire, the like motion, but not strong enough to excite Sense of it self without the attraction of aire by respiration. And this is a possible cause of Smelling.
14 The cause why Smelling is hindred by Cold, and helped by Heat, may be this, that Heat (as hath been shewn in the 21 Chapter) generateth Simple motion; and therefore also wheresoever it is already, there it will encrease it; and the cause of Smelling being encreased the Smell it self will also be encreased. As for the cause why the Wind blowing from the Object makes the Smell the stronger, it is all one with that for which the attraction of aire in respiration doth the same. For he that draws in the aire next to him, draws with it by succession that aire in which is the Object. Now this motion of the aire is Wind, and when another Wind bloweth from the Object, will be encreased by it.
15 That Bodies which co¯tain the least quantity of air, as Stones and Metals, yeild less Smell then Plants and Living Creatures, the cause may be, that the motion which causeth Smelling, is a motion of the fluid parts onely; which parts, if they have any motion from the hard parts in which they are contained, they communicate the same to the open aire, by which it is propagated to the Organ. Where therefore there are no fluid parts, as in Metals; or where the fluid parts receive no motion from the hard parts, as in Stones, which are made hard by accretion, there can be no Smell. And therefore also the Water, whose parts have little or no motion, yeildeth no Smell. But if the same Water, by Seeds, and the heat of the Sunne, be together with particles of Earth raised into a Plant, and be afterwards pressed out again, it will be Odorous, as Wine from the Vine. And as Water passing through plants is by the motion of the parts of those plants made an Odorous liquour; so also of aire passing through the same plants whilest they are growing, are made Odorous aires. And thus also it is with the Juices and Spirits which are bred in Living Creatures.
16 That Odorous Bodies may be made more Odorous by Contrition, proceeds from this, that being broken into many parts which are all Odorous, the aire which by respiration is drawn from the Object towards the Organ, doth in its passage touch upon all those parts, and receives their motion. Now the aire toucheth the superficies onely; and a Body having less superficies whilest it is whole, then all its parts together have after it is reduced to powder, it follows that the same Odorous Body yeildeth less Smell whilest it is whole, then it will do after it is broken into smaller parts. And thus much of Smels.
17 The Tast follows; whose generation hath this difference from that of the Sight, Hearing and Smelling, that by these we have Sense of remote Objects; whereas we Tast nothing but what is contiguous, and doth immediately touch either the Tongue or Palate, or both. From whence it is evident, that the cuticles of the Tongue and Palate, and the Nerves inserted into them are the first Organ of Tast; and (because from the concussion of the parts of these, there followeth necessarily a concussion of the Pia Mater) that the action communicated to these, is propagated to the Brain, and from thence to the farthest Organ, namely, the Heart; in whose reaction consisteth the nature of Sense.
Now that Savours (as well as Odours) doe not onely move the Brain, but the Stomack also, as is manifest by the loathings that are caused by them both, they that consider the Organ of both these Senses will not wonder at all; seeing the Tongue, the Palate & the Nostrils have one and the same continued cuticle, derived from the Dura Mater.
And that Effluviums have nothing to doe in the Sense of Tasting, is manifest from this, that there is no Tast where the Organ and the Object are not contiguous.
By what variety of motions the different kinds of Tasts (which are innumerable) may be distinguished, I know not. I might (with others) derive them from the divers figures of those Atomes, of which whatsoever may be Tasted consisteth; or from the diverse motions which I might (by way of Supposition) attribute to those Atomes; conjecturing (not without some likelyhood of truth) that such things as tast Sweet, have their particles moved with slow circular motion, and their figures Spherical, which makes them smooth and pleasing to the Organ; that Bitter things have circular motion, but vehement, and their figures full of Angles, by which they trouble the Organ; and that Sowre things have straight and reciprocal motion, and their figures long and small, so that they cut and wound the Organ. And in like manner I might assigne for the causes of other Tasts such several motions and figures of Atomes as might in probability seem to be the true causes. But this would be to revolt from Philosophy to Divination.
18 By the Touch, we feel what Bodies are Cold or Hot, though they be distant from us. Others, as Hard, Soft, Rough and Smooth, we cannot feel, unless they be contiguous. The Organ of Touch, is every one of those membranes, which being continued from the Pia Mater, are so diffused throughout the whole Body, as that no part of it can be pressed, but the Pia Mater is pressed together with it. Whatsoever therefore presseth it, is felt as Hard or Soft, that is to say, as more or less Hard. And as for the Sense of Rough, it is nothing else but innumerable perceptions of Hard and Hard succeeding one another by short intervals both of time and place. For we take notice of Rough and Smooth, as also of Magnitude and Figure, not onely by the Touch, but also by Memory. For though some things are touched in one Point; yet Rough and Smooth, like Quantity and Figure, are not perceived but by the Flux of a Point, that is to say, we have no Sense of them without Time; and we can have no Sense of Time, without Memory.
CHAP. XXX. OF GRAVITY.
1 A THICK Body doth not contain more Matter (unless also more Place) then a Thinne.
2 That the Descent of Heavy Bodies proceeds not from their own Appetite; but from some Power of the Earth.
3 The difference of Gravities proceedeth from the difference of the Impetus with which the Elements whereof Heavy Bodies are made do fall vpon the Earth.
4 The cause of the Descent of Heavy Bodies.
5 In what proportion the Descent of Heavy Bodies is accelerated.
6 Why those that Dive do not when they are under Water, feel the waight of the Water above them.
7 The Waight of a Body that floateth, is equal to the Waight of so much Water as would fill the space which the immersed part of the Body takes up within the Water.
8 If a Body be Lighter then Water, then how big soever that Body be, it will float upon any quantity of Water, how little soever.
9 How Water may be lifted up and forced out of a Vessel by Air.
10 Why a Bladder is Heavier when blown full of aire, then when it is empty.
11 The cause of the ejection upwards of Heavy Bodies from a Wind-Gun.
12 The cause of the ascent of Water in a Weather-glass.
13 The cause of motion upwards in Living Creatures.
14 That there is in Nature a kind of Body Heavier then Aire, which nevertheless is not by Sense distinguishable from it.
15 Of the cause of Magnetical vertue.
1 IN the 21 Chapter I have defined Thick and Thinne (as that place required) so, as that by Thick was signified a more Resisting Body, and by Thinne a Body less Resisting; following the custome of those that have before me discoursed of Refraction. Now if we consider the true and vulgar signification of those words, we shall find them to be Names Collective, that is to say, Names of Multitude; as Thick to be that which takes up more parts of a space given, & Thinne that which contains fewer parts of the same magnitude, in the same space, or in a space equal to it. Thick therefore is the same with Frequent, as a Thick Troop; And Thinne the same with Unfrequent, as a Thinne Rank, Thinne of Houses; not that there is more matter in one place then in another equal place, but a greater quantity of some named Body. For there is not less matter or Body indefinitely taken, in a Desert, then there is in a City; but fewer Houses, or fewer Men. Nor is there in a Thick Rank a greater quantity of Body, but a greater number of Souldiers, then in a Thinne. Wherefore the multitude & paucity of the parts contained within the same space, do constitute Density and Rarity, whether those parts be separated by Vacuum, or by Aire. But the consideration of this is not of any great moment in Philosophy; and therefore I let it alone, and pass on to the search of the causes of Gravity.
2 Now we call those Bodies Heavy, which (unless they be hindred by some force) are carried towards the center of the Earth, and that by their own accord, for ought we can by Sense perceive to the contrary. Some Philosophers therefore have been of opinion, that the Descent of Heavy Bodies proceeded from some internal Appetite, by which when they were cast upwards, they descended again, as moved by themselves, to such place as was agreeable to their nature. Others thought they were attracted by the Earth. To the former I cannot assent, because I think I have already clearly enough demonstrated, that there can be no beginning of motion, but from an external & moved Body; and consequently, that whatsoever hath motion or endeavour towards any place, will alwayes move or endeavour towards that same place, unless it be hindered by the reaction of some external Body. Heavy Bodies therefore being once cast upwards, cannot be cast down again but by external motion. Besides, seeing inanimate Bodies have no Appetite at all, it is ridiculous to think that by their own innate Appetite they should to preserve themselves (not understanding what preserves them) forsake the place they are in, and transferre themselves to another place; whereas Man (who hath both Appetite and understanding) cannot for the preservation of his own life, raise himselfe by leaping above three or four feet from the ground. Lastly, to attribute to created Bodies the power to move themselves, what is it else then to say that there be creatures which have no dependance upon the Creator? To the later, who attribute the Descent of Heavy Bodies to the attraction of the Earth, I assent. But by what motion this is done, hath not as yet been explained by any man. I shall therefore in this place say somewhat of the manner, and of the way by which the Earth by its action attracteth Heavy Bodies.
3 That by the supposition of simple motion in the Sunne, homogeneous Bodies are congregated, and heterogeneous dissipated, has already been demonstrated in the 5th Article of the 21 Chapter. I have also supposed, that there are intermingled with the pure Air, certain little Bodies, or (as others call them) Atomes, which by reason of their extreme smalness are invisible, and differing from one another in Consistence, Figure, Motion & Magnitude; from whence it comes to pass, that some of them are congregated to the Earth, others to other Planets, and others are carried up and down in the spaces between. And seeing those which are carried to the Earth, differ from one another in Figure, Motion and Magnitude, they will fall upon the Earth, some with greater, others with less Impetus. And seeing also that we compute the several degrees of Gravity no otherwise then by this their falling upon the Earth with greater or less Impetus; it follows, that we conclude those to be the more Heavy that have the greater Impetus, and those to be less Heavy that have the less Impetus. Our enquiry therefore must be, by what means it may come to pass, that of Bodies which descend from above to the Earth, some are carried with greater, others with less Impetus; that is to say, some are more Heavy then others. We must also enquire, by what means such Bodies as settle upon the Earth, may by the Earth it self be forced to ascend.
4 Let the Circle made upon the center C (in the 2d figure) be a great Circle in the Superficies of the Earth, passing through the points A and B. Also let any Heavy Body, as the stone A D be placed any where in the plain of the Aequator; and let it be conceived to be cast up from A D perpendicularly, or to be carried in any other line to E, and supposed to rest there. Therefore how much space soever the stone took up in A D, so much space it takes up now in E. And because all place is supposed to be full, the space A D will be filled by the aire which flows into it first from the neerest places of the Earth, and afterwards successively from more remote places. Upon the center C let a Circle be understood to be drawn through E; and let the plain space which is between the Superficies of the Earth and that Circle, be divided into plain Orbs equal and concentrique; of which, let that be the first which is contained by the two perimeters that pass through A & D. Whilst therefore the aire which is in the first Orbe, filleth the place A D, the Orbe it self is made so much less, and consequently its latitude is less then the straight line A D. Wherefore there will necessarily descend so much aire from the Orbe next abvoe. In like manner, for the same cause, there will also be a descent of aire from the Orbe next above that; and so by Succession from the Orbe in which the Stone is at rest in E. Either therefore the Stone it self, or so much aire will descend. And seeing aire is by the diurnal revolution of the Earth more easily thrust away, then the Stone, the aire which is in the Orbe that contains the Stone will be forced further upwards then the Stone. But this, without the admission of Vacuum, cannot be, unless so much aire descend to E from the place next above; which being done, the Stone will be thrust downwards. By this means therefore the Stone now receives the beginning of its Descent, that is to say, of its Gravity. Furthermore, whatsoever is once moved, will be moved continually (as hath been shewn in the 19th Article of the 8th Chapter) in the same way, and with the same celerity, except it be retarded or accelerated by some external Movent. Now the aire (which is the onely Body that is interposed between the Earth A and the stone above it E) will have the same action in every point of the straight line E A, which it hath in E. But it depressed the stone in E; and therefore also it will depress it equally in every point of the straight line E A. Wherefore the stone will descend from E to A with accelerated motion. The possible cause therefore of the Descent of Heavy Bodies under the Aequator, is the Diurnal motion of the Earth. And the same demonstration will serve, if the stone be placed in the plain of any other Circle parallel to the Aequator. But because this motion hath by reason of its greater slowness, less force to thrust off the aire in the parallel Circles then in the Aequator, and no force at all at the Poles, it may well be thought (for it is a certain consequent) that Heavy Bodies descend with less and less velocity, as they are more & more remote from the Aequator; & that at the Poles themselves they wil either not descend at all, or not descend by the Axis; which whether it be true or false, Experience must determine. But it is hard to make the experiment, both because the times of their descents cannot be easily measured with sufficient exactness, and also because the places neer the Poles are inaccessible. Nevertheless, this we know, that by how much the neerer we come to the Poles, by so much the greater are the Flakes of the Snow that falls; and by how much the more swiftly such Bodies descend as are fluid and dissipable, by so much the smaller are the particles into which they are dissipated.
5 Supposing therefore this to be the cause of the Descent of Heavy Bodies; it will follow, that their motion will be accelerated in such manner, as that the spaces which are transmitted by them in the several times, will have to one another the same proportion which the odd numbers have in succession from Unity. For if the straight line EA be divided into any number of equal parts, the Heavy Body descending, will (by reason of the perpetual action of the Diurnal motion) receive from the aire in every one of those times, in every several point of the streight line EA, a several new and equal impulsion; and therefore also in every one of those times it will acquire a several and equal degree of celerity. And from hence it follows, by that which Galilaeus hath in his Dialogues of Motion demonstrated, that Heavy Bodies descend in the several times with such differences of transmitted spaces as are equal to the differences of the square numbers that succeed one another from Unity; which square numbers being 1, 4, 9, 16, &c. their differences are 3, 5, 7, &c; that is to say, the odd numbers which succeed another from Unity. Against this cause of Gravity which I have given, it will perhaps be objected, that if a Heavy Body be placed in the bottom of some hollow Cylinder of Iron or Adamant, and the bottom be turned upwards, the Body will descend, though the aire above cannot depress it, much less accelerate its motion. But it is to be considered, that there can be no Cylinder or Cavern, but such as is supported by the Earth, and being so supported, is together with the Earth carried about by its diurnal Motion. For by this means the bottom of the Cylinder will be as the Superficies of the Earth; and by thrusting off the next and lowest aire, will make the uppermost aire depress the Heavy Body which is at the top of the Cylinder, in such manner as is above explicated.
6 The Gravity of Water being so great as by experience wee find it is, the reason is demanded by many, why those that Dive, how deep soever they go under water, do not at all feel the weight of the water which lyes upon them. And the cause seems to be this, that all Bodies by how much the Heavier they are, by so much the greater is the endeavour by which they tend downwards. But the Body of a Man is Heavier then so much water as is equal to it in magnitude, and therefore the endeavour downwards of a Mans Body is greater then that of water. And seeing all endeavour is motion, the Body also of a Man will be carried towards the bottom with greater Velocity then so much water. Wherefore there is greater Reaction from the bottom; and the Endeavour upwards is equal to the endeavour downwards, whether the water be pressed by water, or by another Body which is Heavier then water. And therefore by these two opposite equal endeavours, the endeavour both ways in the water is taken away; and consequently, those that Dive are not at all pressed by it.
Coroll. From hence also it is manifest, that water in water hath no Waight at all, because all the parts of water (both the parts above, and the parts that are directly under) tend towards the bottom with equal endeavour and in the same straight lines.
7 If a Body float upon the water, the waight of that Body is equal to the waight of so much water as would fill the place which the immersed part of the Body takes up within the water.
Let EF (in the 3d figure) be a Body floating in the water ABCD; and let the part E be above, and the other part F under the water. I say the waight of the whole Body EF is equal to the waight of so much water as the Space F will receive. For seeing the waight of the Body EF forceth the water out of the space F, and placeth it upon the Superficies AB, where it presseth downwards; it follows, that from the resistance of the bottom there will also be an endeavour upwards. And seeing again that by this endeavour of the water upwards, the Body EF is lifted up; it follows, that if the endeavour of the Body downwards be not equal to the endeavour of the water upwards, either the whole Body EF will (by reason of that inequality of their endeavours or moments) be raised out of the water, or else it will descend to the bottom. But it is supposed to stand so, as neither to ascend nor descend. Wherefore there is an Aequilibrium between the two endeavours; that is to say, the waight of the Body EF is equal to the waight of so much water as the Space F will receive; Which was to be proved.
8 From hence it follows, that any Body of how great magnitude soever, provided it consist of matter less Heavy then water, may nevertheless float upon any quantity of water how little soever.
Let ABCD (in the 4th figure) be a vessel; and in it let EFGH be a Body consisting of matter which is less Heavy then water; and let the space AGCF be filled with water. I say the Body EFGH will not sink to the bottom DC. For seeing the matter of the Body EFGH is less Heavy then Water, if the whole space without ABCD were full of Water, yet some part of the Body EFGH, as EFIK would be above the Water; and the waight of so much water as would fill the space IGHK would be equal to the waight of the whole Body EFGH; and consequently GH would not touch the bottom DC. As for the sides of the vessel, it is no matter whether they be hard, or fluid; for they serve onely to terminate the Water; which may be done as well by water as by any other matter how hard soever; and the water without the Vessel is terminated somewhere, so as that it can spread no further. The part therefore EFIG will be extant above the water AGCF which is contained in the vessel. Wherefore the Body EFGH will also float upon the water AGCF, how little soever that water be; which was to be demonstrated.
9 In the 4th Article of the 26th Chapter, there is brought for the proving of Vacuum, the experiment of water enclosed in a vessel; which water, the Orifice above being opened, is ejected upwards by the impulsion of the aire. It is therefore demanded (seeing water is Heavier then aire) how that can be done. Let the 2d. figure of the same 26th Chap. be considered, where the water is with great force injected by a Syringe into the space FGB. In that injection, the aire (but pure aire) goeth with the same force out of the vessel through the injected water. But as for those small Bodies which formerly I supposed to be intermingled with aire, & to be moved with simple motion, they can not together with the oure air penetrate the water; but remayning behind are necessarily thrust together into a narrower place, namely into the space which is above the water FG. The motions therefore of those small Bodies will be less and less free, by how much the quantity of the injected water is greater and greater; so that by their motions falling upon one another the same small Bodies will mutually compress each other, and have a perpetual endeavour of regayning their liberty, and of depressing the water that hinders them. Wherefore, as soone as the orifice above is opened, the water which is next it will have an endeavour to ascend; and will therefore necessarily go out. But it cannot go out, unless at the same time there enter in as much aire; and therefore both the water will go out, and the aire enter in, till those small Bodies which were left within the vessel have recovered their former liberty of motion; that is to say, till the vessel be again filled with aire, and no water be left of sufficient height to stop the passage at B. Wherefore I have shewn a possible cause of this Phaenomenon; namely, the same with that of Thunder. For as in the generation of Thunder, the small Bodies enclosed within the Clouds by being too closely pent together, do by their motion break the Clouds, and restore themselves to their natural liberty; so here also the small Bodies enclosed within the space which is above the straight line FG, do by their own motion expel the water as soon as the passage is opened above. And if the passage be kept stopped, and these small Bodies be more vehemently compressed by the perpetual forcing in of more water, they will at last break the vessel it self with great noise.
10 If Aire be blown into a hollow Cylinder, or into a Bladder, it will encrease the waight of either of them a little, as many have found by experience, who with great accurateness have tried the same. And it is no wonder, seeing (as I have supposed) there are intermingled with the common aire a great number of small hard Bodies which are Heavier then the pure aire. For the aethereal substance, being on all sides equally agitated by the motion of the Sunne, hath an equal endeavour towards all the parts of the Universe; and therefore it hath no Gravity at all.
11 We find also by experience, that by the force of air enclosed in a hollow Canon, a bullet of lead may with considerable violence be shot out of a Gunne of late invention, called the Wind-Gun. In the end of this Canon there are two holes with their Valves on the inside, to shut them close; one of them serving for the admission of aire, and the other for the letting of it out. Also to that end which serves for the receiving in of aire, there is joyned another Canon of the same metal and bigness, in which there is fitted a Rammer, which is perforated, and hath also a Valve opening towards the former Canon. By the help of this Valve the Rammer is easily drawn back, and letteth in aire from without; and being often drawn back and returned again with violent strokes, it forceth some part of that aire into the former Canon, so long, till at last the resistance of the enclosed aire is greater then the force of the stroke. And by this means men think there is now a greater quantity of aire in the Canon then there was formerly, though it were full before. Also the aire thus forced in, how much soever it be, is hindered from getting out again by the foresaid Valves, which the very endeavour of the aire to get out doth necessarily shut. Lastly, that Valve being opened which was made for the letting out of the aire, it presently breaketh out with violence, & driveth the bullet before it with great force and velocity.
As for the cause of this, I could easily attribute it (as most men do) to Condensation, and think that the aire, which had at the first but its ordinary degree of Rarity, was afterwards by the forcing in of more aire condensed, and last of all rarified again by being let out and restored to its natural liberty. But I cannot imagine how the same place can be alwayes full, and nevertheless contain sometimes a greater, sometimes a less quantity of matter; that is to say, that it can be fuller then full. Nor can I conceive, how Fulness can of it self be an efficient cause of motion. For both hese are impossible. Wherefore we must seek out some other possible cause of this Phaenomenon. Whilst therefore the Valve wch serves for the letting in of aire is opened by the first stroke of the Rammer, the aire within doth with equal force resist the entering of the aire from without; so that the endeavours between the internal and external aire are opposite, that is, there are two opposite motions, whilest the one goeth in and the other cometh out; but no augmentation at all of aire within the Canon. For there is driven out by the stroke as much pure aire which passeth between the Rammer and the sides of the Canon, as there is forced in of aire impure by the same stroke. And thus by many forcible strokes the quantity of small hard Bodies will be encreased within the Canon, and their motions also will grow stronger and stronger as long as the matter of the Canon is able to endure their force; by which if it be not broken, it will at least be urged every way by their endeavour to free themselves; and as soon as the Valve which serves to let them out is opened, they will fly out with violent motion, and carry with them the bullet which is in their way. Wherefore I have given a possible cause of this Phaenomenon.
12 Water, contrary to the custome of Heavy Bodies, ascendeth in the Weather-glasse; but it doth it when the aire is cold; for when it is warme it descendeth againe. And this Organ is called a Thermometer, or Thermoscope, because the degrees of Heat and Cold are measured and marked by it. It is made in this manner. Let A B C D (in the 5th figure) be a vessel full of water, and E F G a hollow Cylinder of glasse, closed at E, and open at G. Let it be heated, and set upright within the water to F; and let the open end reach to G. This being done as the aire by little and little grows colder, the water will ascend slowly within the Cylinder from F towards E; till at last the externall and internall aire coming to be both of the same temper, it will neither ascend higher, nor descend lower, till the temper of the aire be changed. Suppose it therefore to be setled any where, as at H. If now the heat of the aire be augmented, the water will descend below H; and if the heat be diminished, it will ascend above it. Which though it be certainely known to be true by experience, the cause neverthelesse hath not as yet been discovered.
In the 6 and 7 articles of the 27th chapter (where I consider the cause of Cold) I have shewne, that fluid Bodies are made colder by the pressure of the aire, that is to say, by a constant Wind that presseth them. For the same cause it is, that the Superficies of the water is pressed at F; and having no place to which it may retire from this pressure besides the cavity of the Cylinder between H and E, it is therefore necessarily forced thither by the Cold, and consequently it ascendeth more or lesse, according as the Cold is more or lesse encreased. And againe, as the Heat is more intense, or the Cold more remisse, the same water will be depressed more or lesse by its own Gravity, that is to say, by the cause of Gravity above explicated.
13 Also Living creatures, though they be Heavy, can by Leaping, Swimming & Flying raise themselvs to a certain degree of height. But they cannot do this except they be supported by some resisting Body, as the Earth, the Water and the Aire. For these motions have their beginning from the contraction (by the helpe of the Muscles) of the Body animate. For to this contraction there succeedeth a distension of their whole Bodies; by which distension, the Earth, the Water or the Aire which supporteth them, is pressed; and from hence, by the reaction of those pressed Bodies, Living Creatures acquire an endeavour upwards, but such, as by reason of the Gravity of their Bodies is presently lost againe. By this endeavour therefore it is, that Living creatures rayse themselues up a little way by Leaping, but to no great purpose; but by Swimming & Flying they raise themselves to a greater height; because before the effect of their endeavour is quite extinguished by the Gravity of their bodies, they can renew the same endeavour againe.
That by the power of the Soule, without any antecedent contraction of the Muscles, or the helpe of something to support him, any man can be able to raise his Body upwards, is a childish conceipt. For if it were true, a man might raise himselfe to what height he pleased.
14 The diaphanous Medium which surrounds the Eie on all fides, is invisible; Nor is Aire to be seen in Aire, nor Water in Water, nor any thing but that which is more opacous. But in the confines of two diaphanous Bodies, one of them may be distinguished from the other. It is not therefore a thing so very ridiculous for ordinary people to think all that Space empty, in which we say is Aire; it being the worke of Reason to make us conceive that the Aire is any thing. For by which of our Senses is it, that we take notice of the Aire, seeing we neither See, nor Hear, nor Tast, nor Smell, nor Feel it to be any thing? When we feel Heat, we do not impute it to the Air, but to the Fire; nor do we say the aire is Cold, but we our selves are Cold; and when we feel the Wind, we rather think something is comming, then that any thing is already come. Also we do not at al feel the waight of water in water, much less of air in air. That we come to know that to be a Body which we call Aire, it is by Reasoning; but it is from one Reason onely, namely, because it is impossible for remote Bodies to work upon our Organs of Sense but by the help of Bodies intermediate, without which we could have no sense of them, till they came to be contiguous. Wherefore, from the Senses alone, without reasoning from effects, we cannot have sufficient evidence of the nature of Bodies.
For there is under-ground in some Mines of Coles, a certain matter of a middle nature between Water and Aire, which nevertheless cannot by Sense be distinguished from aire; for it is as Diaphanous as the purest aire; and as farre as Sense can judge, equally penetrable. But if we look upon the effect, it is like that of water. For when that matter breaks out of the Earth into one of those Pits, it fils the same either totally, or to some degree; and if a Man, or Fire be then let down into it, it extinguishes them in almost as little time as water would do. But for the better understanding of this Phaenomenon, I shall describe the 6th figure. In which, let A B represent the pit of the Mine; and let part thereof, namely, C B be supposed to be filled with that matter. If now a lighted Ca¯dle be let down into it below C, it wil as suddenly be extinguished, as if it were thrust into water. Also if a grate filled with coles throughly kindled and burning never so brightly, be let down; as soon as ever it is below C, the fire will begin to grow pale, and shortly after (losing its light) be extinguished, no otherwise then if it were quenched in water. But if the grate be drawn up again presently, whilest the coles are still very hot, the fire will by little and little be kindled again, and shine as before. There is indeed between this matter & water this considerable difference, that it neither wetteth, nor sticketh to such things as are put down into it, as water doth; which by the moisture it leaveth, hindereth the kindling again of the matter once extinguished. In like manner, if a Man be let down below C, he will presently fall into a great difficulty of breathing, and immediately after into a swoun, and die, unless he be suddenly drawn up again. They therefore that go down into these pits, have this custome, that as soon as ever they feel themselves sick, they shake the rope by which they were let down, to signifie they are not well, and to the end that they may speedily be pulled up again. For if a man be drawn out too late, void of sense and motion, they digg up a Turff, and put his face and mouth into the fresh earth; by which means (unless he be quite dead) he comes to himself again by little and little, and recovers life by the breathing out (as it were) of that suffocating matter which he had sucked in whilest he was in the pit; almost in the same manner as they that are drowned come to themselves again by vomiting up the water. But this doth not happen in all Mines, but in some onely; and in those not alwayes, but often. In such Pits as are subject to it, they use this remedy. They dig another pit, as DE, close by it, of equal depth; and joyning them both together with one common channel EF, they make a Fire in the bottom E, which carries out at D the aire contained in the pit DE; and this draws with it the aire contained in the channel EF; which in like manner is followed by the noxious matter contained in CB; & by this means the pit is for that time made healthful. Out of this History (which I write onely to such as have had experience of the truth of it, without any designe to support my Philosophy with Stories of doubtful credit) may be collected the following possible cause of this Phaenomenon; namely, that there is a certain matter, fluid, & most transparent, and not much lighter then water; which breaking out of the Earth fills the Pit to C; and that in this matter, as in water, both Fire and Living creatures are extinguished.
15 About the nature of Heavy Bodies, the greatest difficulty ariseth from the contemplation of those things which make other Heavy Bodies ascend to them; such are Jet, Amber, and the Loadstone. But that which troubles men most is the Loadstone, which is also called Lapis Herculeus; a stone, though otherwise despicable, yet of so great power, that it taketh up Iron from the Earth, and holds it suspended in the aire, as Hercules did Antaeus. Nevertheless, we wonder at it somewhat the less, because we see Jet draw up Straws, which are Heavy Bodies, though not so Heavy as Iron. But as for Jet, it must first be excited by rubbing, that is to say, by motion to and fro; whereas the Loadstone hath sufficient excitation from its own nature, that is to say, from some internal principle of motion peculiar to it self. Now whatsoever is moved, is moved by some contiguous and moved Body, as hath been formerly demonstrated. And from hence it follows evidently, that the first endeavour which Iron hath towards the Loadstone, is caused by the motion of that aire which is contiguous to the Iron. Also that this motion is generated by the motion of the next aire, and so on successively, till by this succession we find that the motion of all the intermediate air taketh its beginning from some motion which is in the Loadstone it self; which motion (because the Loadstone seems to be at rest) is invisible. It is therefore certain, that the attractive power of the Loadstone is nothing else but some motio¯ of the smallest particles thereof. Supposing therefore that those small Bodies of which the Loadstone is (in the bowels of the Earth) composed, have by nature such motion or endeavour as was above attributed to Jet, namely a reciprocal motio¯ in a line too short to be seen, both those stones wil have one & the same cause of attraction. Now in what manner, and in what order of working this cause produceth the effect of attraction, is the thing to be enquired. And first we know, that when the string of a Lute or Viol is stricken, the Vibration, that is, the reciprocal motion of that string in the same straight Line, causeth like Vibration in another string which has like tension. We know also, that the dregs or small sands which sink to the bottom of a Vessel, will be raised up from the bottom by any strong and reciprocal agitation of the water stirred with the hand or with a staff. Why therefore should not reciprocal motion of the parts of the Loadstone contribute as much towards the moving of Iron? For if in the Loadstone there be supposed such reciprocal motion, or motion of the parts forwards and backwards, it will follow, that the like motion will be propagated by the aire to the Iron, and consequently that there will be in all the parts of the Iron the same reciprocations or motions forwards and backwards. And from hence also it will follow, that the intermediate aire between the Stone and the Iron will by little and little be thrust away; and the aire being thrust away, the Bodies of the Loadstone and the Iron will necessarily come together. The possible cause therefore why the Loadstone and Jet draw to them, the one Iron, the other Strawes, may be this, that those attracting Bodies have reciprocal motion either in a straight line, or in an Elliptical line, when there is nothing in the nature of the attracted Bodies which is repugnant to such a motion.
But why the Loadstone (if with the help of Cork it float at liberty upon the top of the water) should from any position whatsoever so place it self in the plain of the Meridian, as that the same points which at one time of its being at rest respect the Poles of the Earth, should at all other times respect the same Poles, the cause may be this, That the reciprocal motion which I supposed to be in the parts of the Stone, is made in a line parallel to the Axis of the Earth, and has been in those parts ever since the Stone was generated. Seeing therefore the Stone whilest it remains in the Mine, and is carried about together with the Earth by its diurnal motion, doth by length of time get a habit of being moved in a line which is perpendicular to the line of its reciprocal motion, it will afterwards, though its axis be removed from the parallel situation it had with the axis of the Earth, retain its endeavour of returning to that situation again; and all endeavour being the beginning of motion, and nothing intervening that may hinder the same, the Loadstone will therefore return to its former situation. For any piece of Iron that has for a long time rested in the plain of the Meridian, whensoever it is forced from that situation, and afterwards left to its own liberty again, will of it self return to lie in the Meridian again; which return is caused by the endeavour it acquired from the diurnal motion of the Earth in the parallel circles which are perpendicular to the Meridians.
If Iron be rubbed by the Loadstone drawn from one Pole to the other, two things will happen; one, that the Iron will acquire the same direction with the Loadstone, that is to say, that it will lie in the Meridian, and have its Axis and Poles in the same position with those of the Stone; the other, that the like Poles of the Stone and of the Iron will avoid one another, and the unlike Poles approach one another. And the cause of the former may be this, that Iron being touched by motion which is not reciprocal, but drawn the same way from Pole to Pole, there will be imprinted in the Iron also an endeavour from the same Pole to the same Pole. For seeing the Loadstone differs from Iron no otherwise then as Ore from Metal, there will be no repugnance at all in the Iron to receive the same motion which is in the Stone. From whence it follows, that seeing they are both affected alike by the diurnal motion of the Earth, they will both equally return to their situation in the Meridian whensoever they are put fro¯ the same Also of the later this may be the cause, that as the Loadstone in touching the Iro¯ doth by its action imprint in the Iron an endeavour towards one of the Poles, suppose towards the North Pole; so reciprocally, the Iron by its action upon the Loadstone doth imprint in it an endeavour towards the other Pole, namely towards the South Pole. It happens therefore in these reciprocations or motions forwards and backwards of the particles of the Stone and of the Iron betwixt the North & the South, that whilest in one of them the motion is from North to South, and the return from South to North, in the other the motion wil be from South to North, & the return fro¯ North to South; which motions being opposite to one another, and communicated to the Air, the North Pole of the Iron (whilest the attraction is working) will be depressed towards the South Pole of the Loadstone; or contrarily the North Pole of the Loadstone will be depressed towards the South Pole of the Iron; and the Axes both of the Loadstone and of the Iron will be situate in the same straight line. The truth whereof is taught us by experience.
As for the propagation of this Magnetical vertue, not onely through the Aire, but through any other Bodies how hard so ever, it is not to be wondred at, seeing no motion can be so weak, but that it may be propagated infinitely through a space filled with Body of any hardness whatsoever. For in a full Medium, there can be no motion which doth not make the next part yeild, and that the next, and so successively without end; so that there is no effect whatsoever but to the production thereof something is necessarily contributed by the several motions of all the several things that are in the World.
And thus much concerning the nature of Body in general; with which I conclude this my first Section of the Elements of Philosophy. In the first, second and third Parts, where the Principles of Ratiocination consist in our own Understanding, that is to say, in the legitimate use of such Words as we our selves constitute, all the Theoremes (if I be not deceived) are rightly demonstrated. The fourth Part depends upon Hypotheses; which unless we know them to be true, it is impossible for us to demonstrate that those Causes which I have there explicated, are the true Causes of the things whose productions I have derived from them.
Nevertheless, seeing I have assumed no Hypothesis, which is not both possible and easie to be comprehended; and seeing also that I have reasoned aright from those Assumptions, I have withall sufficiently demonstrated that they may be the true Causes; wch is the end of Physical Contemplation. If any other man from other Hypotheses shall demonstrate the same, or greater things, there wil be greater praise and thanks due to him then I demand for my self, provided his Hypotheses be such as are conceivable. For as for those that say any thing may be moved or produced by it Self, by Species, by its own Power, by Substantial Forms, by Incorporeal Substances, by Instinct, by Anteperistasis, by Antipathy, Sympathy, Occult Quality, and other empty words of Schoolmen, their saying so is to no purpose.
And now I proceed to the Phaenomena of Mans Body; Where I shall speak of the Opticks, and of the Dispositions, Affections, and Manners of Men (if it shall please God to give me life), and shew their Causes.