stars cannot but give some value to an hypothesis that will simplify the celestial motions in general. We know that the sun, at the distance of the fixed star, would appear like one of them; and from analogy we conclude the stars to be suns. Now, since the apparent motions of these seven stars may be accounted for, either by supposing them to move just in the manner they appear to do, or else by supposing the sun alone to have a motion in a direction, somehow not far from that above assigned to it, we are no more authorized to suppose the sun at rest, than we are to deny the diurnal motion of the earth, except in this respect, that the proofs of the latter are very numerous, whereas the former rests only on a few though capital testimonies.

LAPLACE

PIERRE SIMON LAPLACE was born in Normandy, March 28, 1749. Before eighteen years old he was a teacher of mathematics at Beaumont, and soon afterwards gained the attention of D'Alembert by a letter to him on the principles of mathematics. From 1770 for many years he was busy with Lagrange in establishing the permanency of the solar system, accounting for its perturbations, and interactions, and showing that all these changes are periodic. His "Mechanics of the Heavens" was a gigantic exposition of the movements of the solar system. In his "System of the World" he advanced the nebular hypothesis of the origin of the universe. Most of his previous work had been done in tracing the law of gravitation throughout its many complications, in the system of planets: this latter hypothesis, though relegated to a note at the end of the "System of the World," was to give astronomy a dynamic rather than a descriptive point of view.

He tried to be a politician, but was not a good man of affairs. His ability to change with the wind, however, brought him a title of count from Napoleon, and of marquis (1817) from the restored Bourbon king. He died March 5, 1827.

THE NEBULAR HYPOTHESIS

From the preceding chapter, it appears that we have the five following phenomena to assist us in investigating the cause of the primitive motions of the planetary system. The motions of the planets in the same direction, and very nearly in the same plane; the motions of the satellites in the same direction as those of the planets; the motions of rotation of these different bodies and also of the sun, in the same direction as their motions of projection, and in planes very little inclined to each other; the small eccentricity of the orbits of the planets and satellites; finally, the great eccentricity of the orbits of the comets, their inclinations being at the same time entirely indeterminate.

Buffon is the only individual that I know of, who, since the discovery of the true system of the world, endeavoured to investigate the origin of the planets and satellites. He supposed that a comet, by impinging on the Sun, carried away a torrent of matter, which was reunited far off, into globes of different magnitudes and at different distances from this star. These globes, when they cool and become hardened, are the planets and their satellites. This hypothesis satisfies the first of the five preceding phenomena; for it is evident that all bodies thus formed should move very nearly in the plane which passes through the centre of the Sun, and through the direction of the torrent of matter which has produced them: but the four remaining phenomena appear to me inexplicable on this supposition. Inded, the absolute motion of the molecules of a planet ought to be in the same direction as the motion of the centre of gravity; but it by no means follows from this, that the motion of rotation of a planet should be also in the same direction. Thus the Earth may revolve from east to west, and yet the absolute motion of each of its molecules may be directed from west to east. This observation applies also to the revolution of the satellites, of which the direction in the same hypothesis, is not necessarily the same as that of the motion of projection of the planets.

The small eccentricity of the planetary orbits is a phenomenon, not only difficult to explain on this hypothesis, but altogether inconsistent with it. We know from the theory of central forces, that if a body which moves in a re-entrant orbit about the Sun, passes very near the body of the Sun, it will return constantly to it, at the end of each revolution.