Page:Travels to Discover the Source of the Nile - In the Years 1768, 1769, 1770, 1771, 1772, and 1773 volume 1.djvu/266

158 half, was not really made by him, but was some old Chaldaic, or Egyptian observation, made by more instructed astronomers which he had fallen upon.

Arabs call it Assouan, which they say signifies enlightened; in allusion, I suppose, to the circumstance of the well, enlightened within by the sun's being stationary over it in June; in the language of Beja its name signifies a circle, or portion of a circle.

, among other things, is famous for the first attempt made by Greek astronomers to ascertain the measure of the circumference of the earth. Eratosthenes, born at Cyrene about 276 years before Christ, was invited from Athens to Alexandria by Ptolemy Evergetes, who made him keeper of the Royal Library in that city. In this experiment two positions were assumed, that Alexandria and Syene were exactly 5000 stades distant from each other, and that they were precisely under the same meridian. Again, it was verified by the experiment of the well, that, in the summer solstice at mid-day, when the sun was in the tropic of Cancer, in its greatest northern declination, the well at that instant was totally and equally illuminated; and that no style, or gnomon, erected on a perfect plane, did cast, or project, any manner of shadow for 150 stades round, from which it was justly concluded, that the sun, on that day, was so exactly vertical to Syene, that the center of its disk immediately corresponded to the center of the bottom of the well. These preliminaries being fixed, Eratosthenes set about his observation thus:—