Page:The geography of Strabo (1854) Volume 1.djvu/24

STRABO. 11. What we have already advanced is sufficient to prove the father^of geography. Those who followed in of Ptolemy Euergetes, who placed him over the library at Alexandria. Here he continued till the reign of Ptolemy Epiphanes. He died at the age of eighty, about B. c. 196, of voluntary starvation, having lost his sight, and being tired of life. He was a man of very extensive learning : we shall first speak of him as a geometer and astronomer. " It is supposed that Eratosthenes suggested to Ptolemy Euergetes the construction of the large armillte, or fixed circular instruments, which were long in use at Alexandria ; but only because it is difficult to imagine to whom else they are to be assigned, for Ptolemy the astronomer, though he mentions them, and incidentally their antiquity, does not state to whom they were due. In these circles each degree Avas divided into six parts. We know of no observations of Eratosthenes in which they were probably employed, except those which led him to the obliquity of the ecliptic, which he must have made to be 23 51' 20"; for he states the distance of the tropics to be eleven times the eighty -third part of the circumference. This was a good observation for the times. Ptolemy the astronomer was content with it, and according to him Hipparchus used no other. Of his measure of the earth we shall presently speak. According to Nicoma- chus, he was the inventor of the KOVKIVOV, or Cribrum Arithmeticum, as it has since been called, being the well-known method of detecting the prime numbers by writing down all odd numbers which do not end with 5, and striking out successively the multiples of each, one after the other, so that only prime numbers remain. " We still possess under the name of Eratosthenes a work, entitled KaraTtpioym, giving a slight account of the constellations, their fabulous history, and the stars in them. It is however acknowledged on all hands that this is not a work of Eratosthenes. * * * The only other writ- ing of Eratosthenes which remains, is a letter to Ptolemy on the dupli- cation of the cube, for the mechanical performance of which he had contrived an instrument, of which he seems to contemplate actual use in measuring the contents of vessels, &c. He seems to say that he has had his method engraved in some temple or public building, with some verses, which he adds. Eutocius has preserved this letter in his comment on book ii. prop. 2, of the sphere and cylinder of Archimedes. " The greatest work of Eratosthenes, and that which must always make his name conspicuous in scientific history, is the attempt which he made to measure the magnitude of the earth, in which he brought forward and used the method which is employed to this day. Whether or no he was successful cannot be told, as we shall see ; but it is not the less true that he was the originator of the process by which we now know, very nearly indeed, the magnitude of our own planet. Delambre says that if it were he who advised the erection of the circular instruments above alluded to, he must be considered as the founder of astronomy : to which it may be added, that he was the founder of geodesy without any if in the case. The number of ancient writers who have alluded to this remarkable oper- ation (which seems to have obtained its full measure of fame) is very great, and we shall not attempt to combine their remarks or surmises : it is enough to say that the most distinct account, and one of the earliest, is found in the remaining work of Cleomedes.