It has been observed since the dawn of science that changes are taking place in the motions of the heavenly bodies. The eccentricity of the earth's orbit has been gradually diminishing from the earliest observations to the present time. The moon, also, has been moving faster and faster from the time of the first recorded eclipses, and is now in advance by about four times her own breadth of what her place would have been had she not been affected by these accelerations.[155] In a few thousand years she will be half a month ahead of the place she would be in if her month were to remain constant. The moon is, therefore, approaching closer and closer to the earth; and if these changes go on uninterruptedly, without any reaction or adjustment, sooner or later the final catastrophe must come, and the moon be precipitated on the body of the earth. Toward the close of the last century, Laplace, in his great work, the "Méchanique Céleste," attempted by certain mathematical computations to show that, nevertheless, the solar system is stable and permanent. The planets, by their mutual attractions, produce perpetual perturbations in one another's movements. Laplace believed he could prove that these were periodic; they reach a maximum value and then diminish, oscillating between very narrow extremes. He therefore taught that the machine would go on by a kind of perpetual motion, without any winding up or adjustment from without; and, consequently, the eternal continuance of the solar system is insured. All the investigations of Laplace, and the computations of Lagrange, proceeded on two assumptions: first, that the planets are moving _in vacuo_; and, secondly, that they are solid throughout their entire mass. The latter assumption is certainly in conflict with well-determined geological facts; and there is no _à priori_ ground for assuming that the planetary spaces are void and empty. On the contrary, the general analogies of nature would lead us to the very opposite conclusion, and all attempts at producing a perfect vacuum have hitherto failed. Furthermore, the great body of modern physicists, and nearly all modern astronomers, hold that the celestial spaces are filled with a "material ether," which must by its very nature offer some resistance to planetary motion.