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Article 1: Of the Formation of the Planets
Our subject being Natural History, we would willingly dispense with astronomical observations; but as the nature of the earth is so closely connected with the heavenly bodies, and such observations being calculated to illustrate more fully what has been said, it is necessary to give some general ideas of the formation, motion, and figure of the earth and other planets.
The earth is a globe of about three thousand leagues diameter; it is situated one thousand millions of leagues from the sun, around which it makes its revolution in three hundred and sixty-five days. This revolution is the result of two forces, the one may be considered as an impulse from right to left, or from left to right, and the other an attraction from above downwards, or beneath upwards, to a common center. The direction of these two forces, and their quantities, are so nicely combined and proportioned, that they produce an almost uniform motion in an ellipse, very near to a circle. Like the other planets the earth is opaque, it throws out a shadow; it receives and reflects the light of the sun, round which it revolves in a space of time proportioned to its relative distance and density. It also turns round its own axis once in twenty-four hours, and its axis is inclined 66 [1/2] degrees on the plane of the orbit. Its figure is spheriodal, the two axes of which differ about [160 1/15th] part from each other, and the smallest axis is that round which the revolution is made.
These are the principal phenomena of the earth, the result of discoveries made by means of geometry, astronomy, and navigation. We shall not here enter into the detail of the proofs and observations by which those facts have been ascertained, but only make a few remarks to clear up what is still doubtful, and at the same time give our ideas respecting the formation of the planets, and the different changes through which it is possible they have passed before they arrived at the state [in which we see them today].
There have been so many systems and hypotheses framed upon the formation of the terrestrial globe, and the changes which it has undergone, that we may presume to add our conjectures to those who have written upon the subject; especially as we mean to support them with a greater degree of probability than has hitherto been done; and we are the more inclined to deliver our opinion upon this subject, from the hope that we shall enable the reader to pronounce on the difference between an hypotheses drawn from possibilities, and a theory founded on facts; between a system such as we are here about to present, on the formation and original state of the earth, and a physical history of its real condition, which has been given in the preceding discourse.
Galileo [discovered] the laws of falling bodies, and Kepler observed, that the area described by the principal planets in moving round the sun, and those of the satellites round the planets to which they belong, are proportional to the time of their revolutions, and that such periods were also in proportion to the square roots of the cubes of their distances from the sun, or principal planets. Newton found that the force which caused heavy bodies to fall on the surface of the earth, extended to the moon, and retained it in its orbit; that this force diminished in the same proportion as the square of the distance increases, and consequently that the moon is attracted by the earth; that the earth and planets are attracted by the sun; and that, in general, all bodies which revolve round a center, and describe areas proportioned to the times of their revolution, are attracted towards that point. This power, known by the name of gravity, is therefore diffused throughout all matter; planets, comets, the sun, the earth, and all nature, is subject to its laws, and it serves as a basis [for] the general harmony which reigns in the universe. Nothing is better proved in physics than the actual [and individualized] existence of this power in every material substance. Observation has confirmed the effects of this power, and geometrical calculations have determined the quantity and relations of it ... .
[One thing causes us pause, and is in fact independent of this theory. This is the force of impulsion.] We evidently see the force of attraction always draws the planets towards the sun, [and] they would fall in a perpendicular line on that planet, if they were not repelled by some other power that obliges them to move in a straight line, and which impulsive force would compel them to fly off the tangents of their respective orbits, if the force of attraction ceased one moment. The force of impulsion was certainly communicated to the planets by the hand of the Almighty, when he gave motion to the universe; but we ought, as much as possible, to abstain in [natural philosophy] from having recourse to supernatural causes; and it appears that a probable reason may be given for this impulsive force, perfectly accordant with the laws of mechanics, and not by any means more astonishing than the changes and revolutions which may and must happen in the universe.
The sphere of the sun’s attraction does not confine itself to the orbs of the planets, but extends to a remote distance, always decreasing in the same ratio as the square of the distance increases; it is demonstrated that the comets which are lost to our sight, in the regions of the sky, obey thispower, and by it their motions, like that of the planets, are regulated. All these stars, whose tracks are so different, move round the sun, and describe areas proportioned to the time; the planets in ellipses more or less approaching a circle, and the comets in narrow ellipses of a great extent. Comets and planets move, therefore, by virtue of the force of attraction and impulsion, which continually acting at one time obliges them to describe these courses; but it must be remarked that comets pass over the solar system in all directions, and that the inclinations of their orbits are very different, insomuch, that although subject, like the planets, to the force of attraction, they have nothing in common with respect to their progressive or impulsive motions, but appear, in this respect, independent of each other: the planets, on the contrary, move round the sun in the same direction, and almost in the same plane, never exceeding [7 1/2] degrees of inclination in their planes, the most distant from their orbits. This conformity of position and direction in the motion of the planets, necessarily implies that their impulsive force has been communicated to them by one and the same cause.
May it not be imagined, with some degree of probability, that a comet falling into the body of the sun, will displace and separate some parts from the surface, and communicate to them a motion of impulsion, insomuch, that the planets may formerly have belonged to the body of the sun, and been detached therefrom by an impulsive force, which they still preserve?
This supposition appears to be at least as well founded as the opinion of Leibnitz, who supposes that the earth and planets have formerly been suns; and his system, of which an account will be given in the fifth article, would have been more comprehensive and more agreeable to probability, if he had raised himself to this idea. We agree with him in thinking that this effect was produced at the time when Moses said that God divided light from darkness; for, according to Leibnitz, light was divided from darkness when the planets were [darkened]; but in our supposition there was a physical separation, since the [opaque matter composing the bodies of the planets was actually separated] from the luminous matter which composes the sun.
This idea of the cause of the impulsive force of the planets will be found much less objectionable, when an estimation is made of the analogies and degrees of probability, by which it may be supported. In the first place, the motion of the planets is in the same direction, from West to East, and therefore, according to calculation it is sixty-four to one that such would not have been the case if they had not been indebted to the same cause for their impulsive forces.
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 [it to be] twenty-four to one, that two planets [would be] found in their most distant planes at the same time, and consequently [245], 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, 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 likelihood, that the planets received their impulsive motion by one single stroke. This probability, 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 perihelion it was not more distant from the sun than a sixteenth part of [the solar] 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 re-encounters it will meet with in its road, and on the retardation it suffers in passing through the atmosphere of the sun.
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 [consumes that star], and the motion of impulsion which they will have communicated to the sun, will produce no other effect than that of [displacing] 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, 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 [the mass of this star].
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 it, it cannot be doubted that comets are composed of extremely solid and dense matters, and that they contain a great 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, proportional 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.
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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 willhave 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 [it], and constrain them to circulate around [its] 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.
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 accords so well with the laws of hydrostatics, necessarily supposes that the earth and planets have been in a fluid state, and I am here in agreement with Mr. Leibnitz. This state of fluidity would be caused by violent heating, and] the internal part of the earth must be a vitrifiable matter, of which sand, granite, etc. are the fragments and scoria.
It may, therefore, with some probability be thought, that the planets [originated from] 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. Itnow only remains, to complete this theory, to explain the diurnal motion of the planets, and the formation of 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. [This is another proof of the separation of parts of greater and lesser density when the planets were formed].
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 around 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.
Translated by J. S. Barr
Reading and Discussion Questions
1.What Newtonian principles can Buffon assume in making his argument about the formation of the solar system?
2.What scenario does Buffon offer for the origin of the solar system, and what evidence does he give for thinking that this hypothesis might be correct?
Additional Resources
Apsell, Paula, Producer. Oxley, Chris, Director. Newton’s Dark Secrets. [Documentary] USA: PBS, 2005. An entertaining introduction to Newton’s physical, alchemical, and religious writings.
Christianson, John Robert. “Brahe, Tycho.” In The Complete Dictionary of Scientific Biography. Vol. 19. Detroit: Charles Scribner’s Sons, 2008, 380–5.
Cohen, I. Bernard. The Birth of a New Physics. 2nd ed. New York: W. W. Norton and Company, 1985. A scholarly and accessible (if occasionally whiggish) presentation of conceptual developments in astronomy. The diagrams are invaluable throughout, and especially helpful in understanding Kepler’s three laws of motion.
Crowe, Michael J. Mechanics from Aristotle to Einstein. Santa Fe, NM: Green Lion Press, 2007. A valuable mixture of primary and secondary source material, the chapter on Galileo is particularly helpful.
Densmore, Dana, ed. Selections from Newton’s Principia. Green Lion Press, 2004. The gold standard of readable introductions to the Principia.
Donahue, William H., ed. Selections from Kepler’s Astronomia Nova. Santa Fe, NM: Green Lion Press, 2004. Kepler’s argument against the literal interpretation of scripture in the introduction is fascinating.
Ferguson, Kitty. Tycho and Kepler: The Unlikely Partnership That Forever Changed Our Understanding of the Heavens. New York: Walker and Company, 2002. A very readable introduction to the lives of these extremely eccentric astronomers.
Gregory, Frederick. Natural Science in Western History. Boston, MA: Houghton Mifflin Company, 2008. A solid secondary source textbook, by a past president of the History of Science Society, for the whole period covered by the readings in this volume.
Kuhn, Thomas. The Structure of Scientific Revolutions. 3rd ed. Chicago: University of Chicago Press, 1996.
Losee, John. A Historical Introduction to the Philosophy of Science. 4th ed. Oxford: Oxford University Press, 2001. Losee’s seventh chapter “The Seventeenth Century Attack on Aristotelian Philosophy” helps put this entire section in context.
Machamer, Peter, ed. The Cambridge Companion to Galileo. Cambridge: Cambridge University Press, 1998. Contains several important essays, but “Galileo on Science and Scripture” by Ernan McMullin provides vital context for understanding Galileo’s trial.
Matthews, Michael R., ed. The Scientific Background to Modern Philosophy: Selected Readings. Indianapolis: Hackett Publishing Co., 1989.