sect, iv.] DISSERTATION SECOND. 165 



except (hat of the mathematical principle on which it was to 

 be determined. The measurement of an arch of the meri- 

 dian was attempted by Eratosthenes of Alexandria, in per- 

 fect conformity with that principle, but by means very 

 inadequate to the importance and difficulty of the problem. 

 By measuring the sun's distance from the zenith of Alexan- 

 dria, on the solstitial day, and by knowing, as he thought 

 he did, that, on the same day, the sun was exactly in the 

 zenith of Syene, he found the distance in the heavens be- 

 tween the parallels of those places to be 7° 12', or a 50th 

 part of the circumference of a great circle. Supposing, 

 then, that Alexandria and Syene were in the same meridian, 

 nothing more was required than to find the distance be- 

 tween them, which, when multiplied by 50, would give the 

 circumference of the globe. The manner in which this 

 was attempted by Eratosthenes is quite characteristick of 

 the infant state of the arts of experiment and observation. 

 He took no trouble to ascertain whether Alexandria and 

 Syene were due north and south of one another : the truth 

 is, that the- latter is considerably east of the former, so 

 that, though their horizontal distance had been accurately 

 known, a considerable reduction would have been necessa- 

 ry, on account of the distance of the one from the meridian 

 of the other. It does not appear, however, that Eratosthe- 

 nes was at any more pains to ascertain the distance than 

 the bearing of the two places. He assumed the former 

 just as it was commonly estimated ; and, indeed, it appears 

 that the distance was not measured till long afterwards, 

 when it was done by the command of Nero. 



It was in this way that the ancients made observations 

 and experiments ; the mathematical principles might be 

 perfectly understood, but the method of obtaining accurate 

 data for the application of those principles was not a subject 

 of attention. The power of resolving the problem was the 



2! 



