It needed but an elementary knowledge of geometry to perceive that correct ideas on this point could be readily obtained by measuring a degree on her surface.

Probably there were early attempts to accomplish this object, the results of which have been lost.

But Eratosthenes executed one between Syene and Alexandria, in Egypt, Syene being supposed to be exactly under the tropic of Cancer.

The two places are, however, not on the same meridian, and the distance between them was estimated, not measured.

Two centuries later, Posidonius made another attempt between Alexandria and Rhodes; the bright star Canopus just grazed the horizon at the latter place, at Alexandria it rose 7 1/2 degrees.