BOOK XVIII. Lvii. 209-211 



necessity of acqiiiring a thorough knowledge of the 

 system of the planets also, wai'ning us to watch the 

 transit of the cold star Saturn. Some people think 

 that butterflies are the most reliable sign of spring, 

 on account of the extremely dehcate structure of that 

 insect ; but in the very year in which I am writing 

 this treatise it has bcen noticed that their supply has 

 becn three times annihilated by a return of cold 

 weather, and that migratory birds arriving on 

 January 27 brought a hope of spring that was soon 

 dashed to the ground by a spell of very severe 

 winter. The procedure is two-fold : first of all it 

 consists in trying to obtain a general principle from 

 celestial phenomena, and then this principle has 

 to be investigated by special signs. Above all 

 there is the variation due to the convexity of the 

 world and the terrestrial gk)be, the same star reveal- 

 ing itself to different nations at a different time, with 

 the consequence that its influence is not operative 

 everywhere on the same days. Additional difficulty 

 has also been caused by authors through their ob- 

 servations having been taken in different regions, and 

 because in the next place they actually pubhsh 

 different results of observations made in the same 

 regions. But there wei'e three main schools, the 

 Chaldaean, the Egyptian and the Greek ; and to 

 these a fourth system was added in our own country 46 b.o. 

 by Caesar during his dictatorship, who with the assis- 

 tance of the learned astronomer Sosigenes brought 

 the separate years back into conformity with the 

 course of the sun — and this theory itself was after- 

 wards corrected (when an error " had becn found), so 

 as to dispense witli an intercalary day for a period 

 of twelve successive years, for the reason that the 



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