274 PHYSICAL SCIENCE BK. vn 



as ours is the most sluggish or the swiftest of 

 motion, whether God causes all things to revolve 

 round us or causes us to revolve. Now, for this 

 it is essential that we have a record of all the 

 appearances of comets in former times. For, on 

 account of their infrequency, their orbit cannot 

 as yet be discovered or examined in detail, to 

 see whether they observe periodic laws, and 

 whether some fixed order causes their reappear- 

 ance at the appointed day. Such a development 

 of astronomy is recent, having been lately intro- 

 duced into Greece. 



Ill 



1 DEMOCRITUS, the most acute of all the ancient 

 philosophers, says he suspects there are several 

 stars whose orbits are erratic. But he has given 

 neither their number nor their names, as the 

 motions of the five planets were not in his time 

 understood. Eudoxus was, in fact, the first to 

 import from Egypt into Greece the knowledge 

 of these motions, though he says nothing about 

 comets. From this it becomes plain that, even 

 among the Egyptians, the people that bestowed 

 most care on observation of the sky, the portion 

 of astronomy that relates to comets had not been 



2 worked out. Subsequently Conon, who was himself 

 a careful investigator, made a record of the sun's 

 eclipses that had been observed by the Egyptians ; 

 but he made no mention of comets, though he 

 would certainly not have omitted anything definite 

 on the subject that he had learned in Egypt. So 

 much is certain ; two authors, Epigenes and Apol- 

 lonius of Myndus, the latter highly skilled in cast- 



