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Ptolemy Euergetes was particularly attentive to the interests of the
library at Alexandria. The first librarian appointed by Ptolemy the
successor of Alexander, was Zenodotus; on his death, Ptolemy Euergetes
invited from Athens Eratosthenes, a citizen of Cyrene, and entrusted to him
the care of the library: it has been supposed that he was the second of
that name, or of an inferior rank in learning and science, because he is
sometimes called Beta; but by this appellation nothing else was meant, but
that he was the second librarian of the royal library at Alexandria. He
died at the age of 81, A.C. 194. He has been called a second Plato, the
cosmographer and the geometer of the world: he is rather an astronomer and
mathematician than a geographer, though geography is indebted to him for
some improvements in its details, and more especially for helping to raise
it to the accuracy and dignity of a science. By means of instruments, which
Ptolemy erected in the museum at Alexandria, he ascertained the obliquity
of the ecliptic to be 23 deg. 51' 20". He is, however, principally celebrated
as the first astronomer who measured a degree of a great circle, and thus
approximated towards the real diameter of the earth.
The importance of this discovery will justify us in entering on some
details respecting the means which this philosopher employed, and the
result which he obtained.
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