220 THE GREEK ASTRONOMY. 



made in this long interval of time ; while his refer- 

 ences to those of Hipparchus are perpetual ; and 

 to those of Aristyllus and Timocharis, and of others, 

 as Conon, who preceded Hipparchus, are not un- 

 frequent. 



This Alexandrian period, so inactive and barren 

 in the history of science, was prosperous, civilized, 

 and literary ; and many of the works which belong 

 to it are come down to us, though those of Hip- 

 parchus are lost. We have the " Uranologion" of 

 Geminus 18 , a systematic treatise on Astronomy, ex- 

 pounding correctly the Hipparchian Theories and 

 their consequences, and containing a good account 

 of the use of the various cycles, which ended in 

 the adoption of the Calippic period. We have 

 likewise "The Circular Theory of the Celestial 

 Bodies" of Cleomedes 19 , of which the principal part 

 is a developement of the doctrine of the sphere, 

 including the consequences of the globular form 

 of the earth. We have also another work on 

 "Spherics" by Theodosius of Bithynia 20 , which con- 

 tains some of the most important propositions of 

 the subject, and has been used as a book of in- 

 struction even in modern times. Another writer 

 on the same subject is Menelaus, who lived some- 

 what later, and whose Three Books on Spherics 

 still remain. 



One of the most important kinds of deduction 

 from a geometrical theory, such as that of the 



18 B. c. 70. 19 B. c. 60. 20 B. c. 50. 



