MECHANICAL PHILOSOPHY. 419 



Galileo, so, about the same time, his theory of the heavenly 

 motions was shown to be baseless. According to Aristotle, it 

 has been already mentioned, the motions of the celestial bodies 

 are circular and uniform because the circle is a perfect figure. 

 Many of his followers, however, attempted to support the doctrine 

 on physical grounds, and asserted with Eudoxus, that the planetary 

 bodies are confined by crystalline spheres, the movements of which 

 gave rise to the motions we perceive. It is somewhat remarkable 

 that the very same bodies, whose motions now tend to establish 

 the existence of a resisting medium in the heavens, should be the 

 first to give evidence against the theory of the crystalline spheres. 

 It was ascertained by Tycho Brahe, from astronomical observations, 

 that the comets were not, as Aristotle supposed, meteors in our 

 atmosphere, but moved throughout the planetary spaces ; and it 

 was plain that, as they crossed the orbits of the planets in every 

 direction, they could have encountered no obstacles such as the 

 spheres of Eudoxus. Yet the doctrine of circular motion remained 

 still undisputed, until Kepler, the successor of Tycho Brahe, 

 announced to the world his great discoveries the elliptic motion of 

 the planets, the equable description of areas, and the harmonic law. 

 It is impossible to read the account given by Kepler of these 

 discoveries, without feeling animated with a portion of his en- 

 thusiasm. " I have stolen," says he, " the golden vases of the 

 Egyptians, to build up a tabernacle for my God, far from the 

 confines of Egypt." But when from these laws of observation he 

 attempts to ascend to their physical causes, his reasonings are 

 deeply tinged with the mysticism which seems to have gained so 

 powerful an ascendancy over his great mind. Of the existence 

 of gravitation, as the ruling principle of the universe, he enter- 

 tained very strong and clear views ; but the attraction itself he 

 conceived to be of the nature of animal force. "If the moon 

 and the earth," he wrote, " were not retained in their orbits by 

 their animal force, or some other equivalent, the earth would 

 mount to the moon by a fifty-fourth part of their distance, and 

 the moon fall towards the earth by the other fifty-three parts, 

 and they would thus meet, the substance of both being assumed 

 to be of the same density. If the earth should cease to attract 

 its waters to itself, all the waters of the sea would be raised, 

 and would flow to the body of the moon." The earth itself he 

 supposed to be an enormous living animal, of which the tides 



