Page:Arrian's Voyage Round the Euxine Sea Translated.djvu/188

138 It does not appear on what grounds Aritotle, or rather the mathematicians of his age, etimated the circumference of the earth to be 400,000 tadia: but this is certain, that Eratothenes, did not borrow his calculations from them, but formed his opinion from obervations of his own, which are yet preerved. He attempted this arduous tak by an actual meaurement of a egment of a great circle on the globe, making his computation upon the whole by uniting obervations made in the heavens with a correponding ditance, meaured (as it was uppoed to be) on a meridian of the earth.

The egment of the meridian, which he fixed on for this purpoe,was that between Alexandria and Syene, the ditance between which places he is aid to have meaured, and found to be 5000 tadia. He alo found that the angle of the meridian hadow upon the caphia or un-dial at Alexandria was equal, at the mmer oitice, to $1⁄50$ part of the circle; and that there was no hadow foom the gnomon at Syene at the ame period of time, and at the ame intant of the day.

Suppoing then Alexandria and Syene to lie under the ame merichan, he concluded that the ditance between them was $1⁄50$ part of a great circle of the earth; and this ditance being 'Will uppoed) by meaure, 5000 tadia, the whole circumference of the earth mut: be of coure 250,000 tadia. But in the account of this proces, which is accurately detailed by Cleomedes, not a word