SEQUEL TO THE EPOCH OF HIPPARCHUS. 225 



from the zenith by a fiftieth part of the circum- 

 ference. These two cities were north and south 

 from each other : and the distance had been deter- 

 mined, by the royal overseers of the roads, to be 

 5000 stadia. This gave a circumference of 250,000 

 stadia to the earth, and a radius of about 40,000. 

 Aristotle 21 says that the mathematicians make the 

 circumference 400,000 stadia. Hipparchus con- 

 ceived that the measure of Eratosthenes ought to 

 be increased by about one tenth 30 . Posidonius, the 

 friend of Cicero, made another attempt of the 

 same kind. At Rhodes, the star Canopus but just 

 appeared above the horizon: at Alexandria, the 

 same star rose to an altitude of ^ th of the cir- 

 cumference ; the direct distance on the meridian 

 was 5000 stadia, which gave 240,000 for the whole 

 circuit. We cannot look upon these measures as 

 very precise ; the stadium employed is not certainly 

 known ; and no peculiar care appears to have been 

 bestowed on the measure of the direct distance. 



When the Arabians, in the ninth century, came 

 to be the principal cultivators of astronomy, they 

 repeated this observation in a manner more suited 

 to its real importance and capacity of exactness. 

 Under the Caliph Almamon 31 , the vast plain of 

 Singiar, in Mesopotamia, was the scene of this un- 

 dertaking. The Arabian astronomers there divided 

 themselves into two bands, one under the direction 



29 De Ccelo. ii. ad fin. 30 Plin. ii. (cviii.) 



31 Montu. i. 357. 



VOL. I. Q 



