Eratosthĕnes

, son of Aglaus, was a native of Cyrene, and the second entrusted with the care of the Alexandrian library. He dedicated his time to grammatical criticism and philosophy, but more particularly to poetry and mathematics. He has been called a second Plato, the cosmographer and the geometer of the world. He is supposed to be the inventor of the armillary sphere. With the instruments with which the munificence of the Ptolemies supplied the library of Alexandria, he was enabled to measure the obliquity of the ecliptic, which he called 20½ degrees. He also measured a degree of the meridian, and determined the extent and circumference of the earth with great exactne , by means adopted by the moderns. He starved himself after he had lived to his 82nd year, B.C. 194. Some few fragments remain of his compositions. He collected the annals of the Egyptian kings by order of one of the Ptolemies. Cicero , Letters to Atticus , bk. 2 , ltr. 6 .— Varro , de Re Rustic a , bk. 1 , ch. 2 .