stars traced. This was the very first occasion upon which a comet had been treated as a celestial body like another. How could an object of the sort circulate among material crystal spheres? Questions of this kind were in men's minds; the observations upon which their solutions must depend were a-making; sufficient progress in mathematics had already been made; the time for a recasting of the accepted theory of the world was at hand.
Crystalline spheres were the basis of the theory of Fracastor. To explain the motions of the heavenly bodies he employed sixty-three spheres whose motions were linked one with another like wheel-work. His doctrine is that: All motions take place in circles; uniform motions are the most probable; each planet always remains at a constant distance from the earth; the changes in their observed brilliancy depend not on changes of distance, but on differences in the earth's atmosphere, or in the density of the crystal spheres; the Primum Mobile moves uniformly and always will do so unless God the Creator intervenes by a special act; spheres are of various kinds—conductors, anti-conductors, circling, anticircling, countervailing; sixty-three of them will explain the world; ten orbs belong to Saturn, eleven to Jupiter, nine to Mars, four to the sun, eleven to Venus, eleven to Mercury, seven to the moon. The system of Fracastor is not only complex, but mechanically impossible. It represents the worst aspect of the doctrine which Copernicus was to overthrow and it is interesting as almost the last exposition of its sort, and especially because Fracastor was a contemporary of Copernicus and died in the same year (1543).
