Part 5

This, probably, will be considerably augmented by the second analogy, viz. that the inclination of the planes of the orbits do not exceed 7-1/2 degrees; for, by comparing the spaces, we shall find there is twenty-four to one, that two planets are found in their most distant places at the same time, and consequently ⁵, or 7,692,624 to one, that all six would by chance be thus placed; or, what amounts to the same, there is a great degree of probability that the planets have been impressed with one common moving force, and which has given them this position. But what can have bestowed this common impulsive motion, but the force and direction of the bodies by which it was originally communicated? It may therefore be concluded, with great probability, that the planets received their impulsive motion by one single stroke. This likelihood, which is almost equivalent to a certainty, being established, I seek to know what moving bodies could produce this effect, and I find nothing but comets capable of communicating a motion to such vast bodies.

By examining the course of comets, we shall be easily persuaded, that it is almost necessary for some of them occasionally to fall into the sun. That of 1680 approached so near, that at its perihelium it was not more distant from the sun than a sixteenth part of its diameter, and if it returns, as there is every appearance it will, in 2255, it may then possibly fall into the sun; that must depend on the rencounters it will meet with in its road, and of the retardment it suffers in passing through the atmosphere of the sun[78:A].

We may, therefore, presume with the great Newton, that comets sometimes fall into the sun; but this fall may be made in different directions. If they fall perpendicular, or in a direction not very oblique, they will remain in the sun, and serve for food to the fire which that luminary consumes, and the motion of impulsion which they will have communicated to the sun, will produce no other effect than that of removing it more or less, according as the mass of the comet will be more or less considerable; but if the fall of the comet is in a very oblique direction, which will most frequently happen, then the comet will only graze the surface of the sun, or slightly furrow it; and in this case it may drive out some parts of matter to which it will communicate a common motion of impulsion, and these parts so forced out of the body of the sun, and even the comet itself, may then become planets, and turn round this luminary in the same direction, and in almost the same plane. We might perhaps calculate what quantity of matter, velocity, and direction a comet should have, to impel from the sun an equal quantity of matter to that which the six planets and their satellites contain; but it will be sufficient to observe here, that all the planets, with their satellites, do not make the 650th part of the mass of the sun,[79:A] because the density of the large planets, Saturn and Jupiter, is less than that of the sun; and although the earth be four times, and the moon near five times more dense than the sun, they are nevertheless but as atoms in comparison with his extensive body.

However inconsiderable the 650th part may be, yet it certainly at first appears to require a very powerful comet to separate even that much from the body of the sun; but if we reflect on the prodigious velocity of comets in their perihelion, a velocity so much the greater as they approach nearer the sun; if, besides, we pay attention to the density and solidity of the matter of which they must be composed, to suffer, without being destroyed, the inconceivable heat they endure; and consider the bright and solid light which shines through their dark and immense atmospheres, which surround, and must obscure them, it cannot be doubted that the comets are composed of extremely solid and dense matters, and that they contain a greater quantity of matter in a small compass; that consequently a comet of no extraordinary bulk may have sufficient weight and velocity to displace the sun, and give a projectile motion to a quantity of matter, equal to the 650th part of the mass of this luminary. This perfectly agrees with what is known concerning the density of planets, which always decreases as their distance from the sun is increased, they having less heat to support; so that Saturn is less dense than Jupiter, and Jupiter much less than the earth; therefore if the density of the planets be, as Newton asserts, proportionable to the quantity of heat which they have to support, Mercury will be seven times more dense than the earth, and twenty-eight times denser than the sun; and the comet of 1680 would be 28,000 times denser than the earth, or 112,000 times denser than the sun, and by supposing it as large as the earth, it would contain nearly an equal quantity of matter to the ninth part of the sun, or by giving it only the 100th part of the size of the earth, its mass would still be equal to the 900th part of the sun. From whence it is easy to conclude, that such a body, though it would be but a small comet, might separate and drive off from the sun a 900th or a 650th part, particularly if we attend to the immense velocity with which comets move when they pass in the vicinity of the sun.

Besides this, the conformity between the density of the matter of the planets, that of the sun deserves some attention. It is well known, that, both on and near the surface of the earth, there are some matters 14 or 1500 times denser than others. The densities of gold and air are nearly in this relation. But the internal parts of the earth and planets are composed of a more uniform matter, whose comparative density varies much less; and the conformity in the density of the planets and that of the sun is such, that of 650 parts which compose the whole of the matter of the planets, there are more than 640 of the same density as the matter of the sun, and only ten parts out of these 650 which are of a greater density, for Saturn and Jupiter are nearly of the same density as the sun, and the quantity of matter which these planets contain, is at least 64 times greater than that of the four inferior planets, Mars, the Earth, Venus, and Mercury. We must therefore admit, that the matter of which the planets are generally composed is nearly the same as that of the sun, and that consequently the one may have been separated from the other.

But it may be said, if the comet, by falling obliquely on the sun, drove off the matter which compose the planets, they, instead of describing circles of which the sun is the centre, would, on the contrary, at each revolution, have returned to the same point from whence they departed, as every projectile would which might be thrown off with sufficient force from the surface of the earth, to oblige it to turn perpetually: for it is easy to demonstrate that such, in that instance, would be the case, and therefore that the projection of the planets from the sun cannot be attributed to the impulsion of a comet.

To this I reply, that the matter which composes the planets did not come from the sun, in ready formed globes, but in the form of torrents, the motion of the anterior parts of which were accelerated by that of the posterior; and that the attraction of the anterior parts also accelerated the motion of the posterior, and that this acceleration produced by one or other of these causes, or perhaps by both, might be so great as to change the original direction of the motion occasioned by the impulse of the comet, from which cause a motion has resulted, such as we at present observe in the planets; especially when it is considered the sun is displaced from its station by the shock of the comet. An example will render this more reasonable; let us suppose, that from the top of a mountain a musket ball is discharged, and that the strength of the powder was sufficient to send it beyond the semi-diameter of the earth, it is certain that this ball would pass round the earth, and at each revolution return to the spot from whence it had been discharged: but, if instead of a musket-ball, we suppose a rocket had been discharged, wherein the action of the fire being durable, would greatly accelerate the motion of impulsion; this rocket, or rather the cartouch which contained it, would not return to the same place like the musket-ball, but would describe an orbit, whose perigee would be much farther distant from the earth, as the force of acceleration would be greater, and have changed the first direction.

Thus, provided there had been any acceleration in the motion of impulsion communicated to the torrent of matter by the fall of the comet, it is probable that the planets formed in this torrent, acquired the motion which we know they have in the circles and ellipsis of which the sun is the centre and focus.

The manner in which the great eruptions of volcanos are made, may afford us an idea of this acceleration of motion. It has been remarked that when Vesuvius begins to roar and eject the inflamed matter it contains, the first cloud has but a small degree of velocity, but which is soon accelerated by the impulse of the second; the second by the action of a third, and so on, until the heavy mass of bitumen, sulphur, cinders, melted metal, and huge stones, appear like massive clouds, and although they succeed each other nearly in the same directions, yet they greatly change that of the first, and drive it far beyond what it would have reached of itself.

In answer to this objection, it may be further observed, that the sun having been struck by the comet, received a degree of motion by the impulse, which displaced it from its former situation; and that although this motion of the sun is at present too little sensible for the notice of astronomers, nevertheless it may still exist, and the sun describe a curve round the centre of gravity of the whole system and if this is so, as I presume it is, we see perfectly that the planets, instead of returning near the sun at each revolution, will, on the contrary, have described orbits, the points of the perihelion of which will be as far distant from the sun, as it is itself from the place it originally occupied.

It may also be said, that if this acceleration of motion is made in the same direction, no change in the perihelion will be produced: but can it be thought that in a torrent, the particles of which succeed each other, there has been no change of direction; it is, on the contrary, very probable that a considerable change did take place, sufficient to cause the planets to move in the course they at present occupy.

It may be further urged, that if the sun had been displaced by the shock of a comet, it would move uniformly, and that hence this motion being common to the whole system, no alteration was necessary; but might not the sun before the shock have had a motion round the centre of the cometry system, to which primitive motion the stroke of the comet may have added or diminished? and would not that fully account for the actual motion of the planets?

If these suppositions are not admitted, may it not be presumed, that in the stroke of the comet against the sun, there was an elastic force which raised the torrent above the surface of the sun, instead of directly impelling it? which alone would be sufficient to remove the perihelion, and give the planets the motion they have retained. This supposition is not without probability, for the matter of the sun may possibly be very elastic, since light, the only part of it we are acquainted with, seems, by its effects, to be perfectly so. I own that I cannot say whether it is by the one or the other of these reasons, that the direction of the first motion of the impulse of the planets has changed, but they suffice to shew that such an alteration is not only possible but even probable, and that is sufficient for my purpose.

But, without dwelling any longer on the objections which might be made, I shall pursue the subject, and draw the fair conclusions on the proofs which analogies might furnish in favour of my hypothesis: let us, therefore, first see what might happen when these planets, and particularly the earth, received their impulsive motion, and in what state they were after having been separated from the sun. The comet having, by a single stroke, communicated a projectile motion to a quantity of matter equal to the 650th part of the sun's mass, the light particles would of course separate from the dense, and form, by their mutual attraction, globes of different densities: Saturn being composed of the most gross and light parts, would be the most remote from the sun: Jupiter being more dense than Saturn, would be less distant, and so on. The larger and least solid planets are the most remote, because they received an impulsive motion stronger than the smallest, and more dense: for, the force of impulsion communicating itself according to the surface, the same stroke would have moved the grosser and lighter parts of the matter of the sun with more velocity than the smallest and more weighty; a separation therefore will be made of the dense parts of different degrees, so that the density of the sun being equal to 100, that of Saturn will be equal to 67, that of Jupiter to 94-1/2, that of Mars to 200, that of the Earth to 400, that of Venus to 800, and that of Mercury to 2800. But the force of attraction not communicating like that of impulsion, according to the surface, but acting on the contrary on all parts of the mass, it will have checked the densest portions of matter; and it is for this reason that the densest planets are the nighest the sun, and turn round that planet with greater rapidity than the less dense planets, which are also the most remote.

Jupiter and Saturn, which are the largest and principal planets of the solar system, have retained the relation between their density and impulsive motions, in the most exact proportions; the density of Saturn is to that of Jupiter as 67 to 94-1/2 and their velocities are nearly as 88-2/3 to 120-1/72, or as 67 to 90-11/16; it is seldom that pure conjectures can draw such exact relations. It is true, that by following this relation between the velocity and density of planets, the density of the earth ought to be only as 206-7/18, and not 400, which is its real density; from hence it may be conceived, that our globe was formerly less dense than it is at present. With respect to the other planets, Mars, Venus, and Mercury, as their densities are known only by conjecture, we cannot be certain whether this circumstance will destroy or confirm our hypothesis. The opinion of Newton is, that density is so much the greater, as the heat to which the planet is exposed is the stronger; and it is on this idea that we have just said that Mars is one time less dense than the Earth, Venus one time, Mercury seven times, and the comet in 1680, 28,000 times denser than the earth: but this proportion between the density of the planets and the heat which they sustain, seems not well founded, when we consider Saturn and Jupiter, which are the principal objects; for, according to this relation between the density and heat, the density of Saturn would be about 4-7/18, and that of Jupiter as 14-17/22, instead of 67 and 94-1/2, a difference too great to be admitted, and must destroy the principles upon which it was founded. Thus, notwithstanding the confidence which the conjectures of Newton merit, I can but think that the density of the planets has more relation with their velocity than with the degree of heat to which they are exposed. This is only a final cause, and the other a physical relation, the preciseness of which is remarkable in Jupiter and Saturn: it is nevertheless true, that the density of the earth, instead of being 206-7/8, is found to be 400, and that consequently the terrestrial globe must be condensed in this ratio of 206-7/8 to 400.

But have not the condensations of the planets some relation with the quantity of the heat of the sun which they sustain? If so, Saturn, which is the most distant from that luminary, will have suffered little or no condensation; and Jupiter will be condensed from 90-11/16 to 94-1/2. Now the heat of the sun in Jupiter being to that of the sun upon the earth as 14-17/22 are to 400, the condensations ought to be in the same proportion. For instance, if Jupiter be condensed, as 90-11/16 to 94-1/2, and the earth had been placed in his orbit, it would have been condensed from 206-7/8 to 215-990/1451, but the earth being nearer the sun, and receiving a heat, whose relation to that which Jupiter receives is from 400 to 14-17/22, the quantity of condensation it would have experienced on the orbit of Jupiter by the proportion of 400 to 14-17/22, which gives nearly 234-1/3 for the quantity which the earth would be condensed. Its density was 206-7/8, by adding the quantity of its acquired condensation, we find 400-7/8 for its actual density, which nearly approaches the real density 400, determined to be so by the parallax of the moon. As to other planets, I do not here pretend to give exact proportions, but only approximations, to point out that their densities have a strong relation to their velocity in their respective orbits.

The comet, therefore, by its oblique fall upon the surface of the sun, having driven therefrom a quantity of matter equal to the 650th part of its whole mass; this matter, which must be considered in a liquid state, will at first have formed a torrent, the grosser and less dense parts of which will have been driven the farthest, and the smaller and more dense, having received only the like impulsion, will remain nearest its source; the force of the sun's attraction would inevitably act upon all the parts detached from him, and constrain them to circulate around his body, and at the same time the mutual attraction of the particles of matter would form themselves into globes at different distances from the sun, the nearest of which necessarily moving with greater rapidity in their orbits than those at a distance.

But another objection may be started, and it may be said, if the matter which composes the planets had been separated from the sun, they, like him, would have been burning and luminous bodies, not cold and opaque, for nothing resembles a globe of fire less than a globe of earth and water; and by comparison, the matter of the earth and planets is perfectly different from that of the sun?

To this it may be answered, that in the separation the matter changed its form, and the light or fire was extinguished by the stroke which caused this motion of impulsion. Besides, may it not be supposed that if the sun, or a burning star, moved with such velocity as the planet, that the fire would soon be extinguished; and that is the reason why all luminous stars are fixed, and that those stars which are called new, and which have probably changed places, are frequently extinguished and lost? This remark is somewhat confirmed by what has been observed in comets; they must burn to the centre when they pass to their perihelium: nevertheless they do not become luminous themselves, they only exhale burning vapours, of which they leave a considerable part behind them in their course.

I own, that in a medium where there is very little or no resistance, fire may subsist and suffer a very great motion without being extinguished: I also own, that what I have just said extends only to the stars which totally disappear, and not to those which have periodical returns, and appear and disappear alternately without changing place in the heavens. The phenomena of these stars has been explained in a very satisfactory manner by M. de Maupertuis, in his discourse on the figures of the planets. But the stars which appear and afterwards disappear entirely, must certainly have been extinguished, either by the velocity of their motion, or some other cause. We have not a single example of one luminous star revolving round another; and among the number of planets which compose our system, and which move round the sun with more or less rapidity, there is not one luminous of itself.

It may also be added, that fire cannot subsist so long in the small as in large masses, and that the planets must have burnt for some time after they were separated from the sun, but were at length extinguished for want of combustible matter, as probably would be the sun itself, and for the same reason; but in a length of time as far beyond that which extinguished the planets, as it exceeds in quantity of matter. Be this as it may, the matter of which the planets are formed being separated from the sun, by the stroke of a comet, that appears a sufficient reason for the extinction of their fires.

The earth and planets at the time of their quitting the sun, were in a state of total liquid fire; in this state they remained only as long as the violence of the heat which had produced it; and which heat necessarily underwent a gradual decay: it was in this state of fluidity that they took their circular forms, and that their regular motions raised the parts of their equators, and lowered their poles. This figure, which agrees so perfectly with the laws of hydrostatics, I am of opinion with Leibnitz, necessarily supposes that the earth and planets have been in a state of fluidity, caused by fire, and that the internal part of the earth must be a vitrifiable matter, of which sand, granite, &c. are the fragments and scoria.

It may, therefore, with some probability, be thought that the planets appertained to the sun, that they were separated by a single stroke, which gave to them a motion of impulsion, and that their position at different distances from the sun proceeds only from their different densities. It now only remains, to complete this theory, to explain the diurnal motion of the planets, and the formation or the satellites; but this, far from adding difficulties to my hypothesis, seems, on the contrary, to confirm it.

For the diurnal motion, or rotation, depends solely on the obliquity of the stroke, an oblique impulse therefore on the surface of a body will necessarily give it a rotative motion; this motion will be equal and always the same, if the body which receives it is homogeneous, and it will be unequal if the body is composed of heterogeneous parts, or of different densities; hence we may conclude that in all the planets the matter is homogeneous, since their diurnal motions are equal, and regularly performed in the same period of time. Another proof that the separation of the dense or less dense parts were originally from the sun.

But the obliquity of the stroke might be such, as to separate from the body of the principal planet a small part of matter, which would of course continue to move in the same direction; these parts would be united, according to their densities, at different distances from the planet, by the force of their mutual attraction, and at the same time follow its course round the sun, by revolving about the body of the planet, nearly in the plane of its orbit. It is plain, that those small parts so separated are the satellites: thus the formation, position, and direction of the motions of the satellites perfectly agree with our theory; for they have all the same motion in concentrical circles round their principal planet; their motion is in the same direction, and that nearly in the plane of their orbits. All these effects, which are common to them, and which depend on an impulsive force, can proceed only from one common cause, which is, impulsive motion, communicated to them by one and the same oblique stroke.