Eratosthenes was contemporary with Archimedes. He was born at Cyrene, B.C. 276. The care of the library appears to have been committed to him by Euergetes; but his attention was more specially directed to mathematical, astronomical, geographical, and historical pursuits. The work entitled "Catasterisms," doubtfully imputed to him, is a catalogue of 475 of the principal stars; but it was probably intended for nothing more than a manual. He also is said to have written a poem upon terrestrial zones. Among his important geographical labours may be mentioned his determination of the interval between the tropics. He found it to be eleven eighty-thirds of the circumference. He also attempted the measurement of the size of the earth by ascertaining the distance between Alexandria and Syene, the difference of latitude between which he had found to be one-fiftieth of the earth's circumference. It was his object to free geography from the legends with which the superstition of ages had adorned and oppressed it. In effecting this he well deserves the tribute paid to him by Humboldt, the modern who of all others could best appreciate his labours. He considered the articulation and expansion of continents; the position of mountain chains; the action of clouds; the geological submersion of lands; the elevation of ancient sea-beds; the opening of the Dardanelles and of the Straits of Gibraltar; the relations of the Euxine Sea; the problem of the equal level of the circumfluous ocean; and the necessary existence of a mountain chain running through Asia in the diaphragm of Dicæarchus. What an advance is all this beyond the meditations of Thales! Herein we see the practical tendencies of the Macedonian wars. In his astronomical observations he had the advantage of using the armils and other instruments in the Observatory. He ascertained that the direction of terrestrial gravity is not constant, but that the verticals converge. He composed a complete systematic description of the earth in three books--physical, mathematical, historical--accompanied by a map of all the parts then known. Of his skill as a geometer, his solution of the problem of two mean proportionals, still extant, offers ample evidence; and it is only of late years that the fragments remaining of his Chronicles of the Theban Kings have been properly appreciated. He hoped to free history as well as geography from the myths that deform it, a task which the prejudices and interests of man will never permit to be accomplished. Some amusing anecdotes of his opinions in these respects have descended to us. He ventured to doubt the historical truth of the Homeric legends. "I will believe in it when I have been shown the currier who made the wind-bags which Ulysses on his homeward voyage received from Æolus." It is said that, having attained the age of eighty years, he became weary of life, and put an end to himself by voluntary starvation. [Sidenote: Chronology of Eratosthenes.]