226 On the time at which Neurchus left the Indus. 



for the 6th of the Ides (that is, the 10th,) of October ; we 

 have therefore the intended sense of our author exhibited in 

 the clearest light/ 1 



After some discussion, Dr Vincent* fixes on the 1st of Oc- 

 tober for the time mentioned by Arrian. Between this and 

 the date derived from Columella there is a considerable differ- 

 ence, which Dr Vincent " never having had leisure to pursue 

 the science of astronomy," was unable to account for. He 

 therefore applied to Mr Wales and Dr Horsley ; and the dis- 

 sertations with which they supplied him are printed at the 

 end of the volume. 



Calculating for the time and country in which Columella 

 lived, Mr Wales found-)- that the point of the ecliptic, which 

 set as the lucida Pleiadum rose, was =£= 29° 7' 9", and in this 

 point the sun then was on the 19th of October. He likewise 

 found that the point which rose as the star set was rri 4° 20', 

 which point the sun occupied on the 29th of October. The 

 former of these dates differs nine days from the time assigned 

 by Columella, which, as we have before- mentioned, was the 

 10th of October ; and the latter differs ten days from his time, 

 which was the 8th of November ; for he says vi. Idus Novem- 

 bris vergiliae mane occidunt. 



Mr Wales examined the effect which the precession of the 

 equinox would have produced between the time of Columella 

 and that of Alexander ; but he found that the alteration would 

 not have amounted to more than two days J, which was not 

 enough to clear up the difficulty. But it may be remarked, 

 as he observes §, that one of these errors is in defect, and the 

 other in excess. And " it is manifest the star's rising cannot 

 be observed when it rises exactly as the sun sets, nor can its 

 setting be seen when it sets exactly as the sun rises, on account 

 of the daylight; but perhaps the one might be seen by a 

 good eye in the latitude of Rome, nine or ten days before, 

 and the other as much after the time when the two circum- 

 stances happened together; and I have not a doubt but that 

 the difference between Columella's observation and my calcu- 

 lation is to be attributed to this cause." 



• P. 196. t P. 501. % P. 503. § P. 502. 



