On the time at which Nearvhus left the Indus 227 



Dr Horsley does not merely reason on Mr Wales's calcula- 

 tions. He goes himself completely through the whole problem, 

 and, making allowance for the effect of refraction, he finds that 

 " *the 19th of October (Styl. Jul.) was the day of the acro- 

 nychal rising of Iucida Pleiadum upon the horizon of the 

 mouth of the Indus in the year before Christ 326." He then 

 discusses the date of the 20th of Boedromion from the Athe- 

 nian Calendar, and says *f*, " it is certain that on one of these 

 two days, either the 30th of September or the 1st of October, 

 Nearchus sailed from the mouth of the Indus, according to 

 Arrian ; consequently, he had been eighteen or nineteen days 

 at sea before the day came of the acronychal rising of Iucida 

 Pleiadum ; taking acronychal rising strictly, according to the 

 mathematical definition of the terms." He goes on to reason 

 on these results, and then says, " after various conjectures 

 and many long calculations, I am entirely persuaded that 

 Mr Wales's very ingenious conjecture, by which he recon- 

 ciles his calculation of the acronychal rising of the Pleiades at 

 Rome, in the year of our Lord 42, with Columella's date, is 

 the only true solution of the difficulty J. It certainly was 

 very natural (and it was the only way for popular use) for the 

 ancients to call that the evening of the acronychal rising, on 

 which they first missed the light of the rising star. 1 ' Dr Hors- 

 ley, however, does not satisfy himself with a loose estimate of 

 the number of days which might intervene between the time 

 at which the star would cease to be visible and that at which it 

 would really set. He reduces it to certainty by calculation. 

 *' Lucida Pleiadum," he adds, " is a star of the third magni- 

 tude; and Ptolemy says, that stars of the third magnitude first 

 become visible when the sun is sunk 14° below the horizon."* 

 Applying this to the given time and place, he found that on 

 the§ 80th of September the star might have been still visible ; 

 " but the next evening, the 1st of October, the sun would be 

 only 13° 37' 15" below the horizon when the star was rising. 

 This evening therefore the star could not be seen upon the 

 horizon. It appears, therefore, that what these mariners 

 would call the acronychal rising of the Pleiades, took place 



* P. 514. f P. 51T* t P. 518. § P 519. 



