Pliny[143] says, that, in his time, the city of Syene was situated so directly under the tropic of Cancer, that there was a well, into which the sun shone so perpendicular, that it was enlightened by its rays down to the bottom. Strabo[144] had said the same. The ignorance, or negligence, in the Geodesique measure in this observation, is extraordinary; Egypt had been measured yearly, from early ages, and the distance between Syene and Alexandria should have been known to an ell. From this inaccuracy, I do very much suspect the other measure Eratosthenes is said to have made, by which he fixed the sun’s parallax at 10 seconds and a 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. The 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. Syene, 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[145] 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:--