342 POPULAR SCIENCE MONTHLY. 



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 solu- 

 tions 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 dis- 

 tance from the earth; the changes in their observed brilliancy depend 

 not on changes of distance, but on differences in the earth's atmos- 

 phere, or in the density of the crystal spheres; the Primum Mobile 

 moves uniformly and always will do so unless God the Creator inter- 

 venes 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 me- 

 chanically 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 con- 

 temporary of Copernicus and died in the same year (1543). 



