Page:Popular Science Monthly Volume 16.djvu/259

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only authority for more than thirteen centuries. Happily it has come down to us entire, though the different manuscript copies vary considerably among themselves. Through the generosity of the Emperor of Russia, facsimile copies have been made of the oldest extant Greek MS., written about the year 1200, and now at Mount Athos; and these copies are in the more important libraries of the world. The method of projection which Ptolemy used in his maps had been slow of practical realization. Hipparchus's work had not gained a circulation. Marin of Tyre had had but poor success in attempting it, so that Ptolemy's approach to success seems the more commendable. We present a map drawn upon his system. In it are to be noticed two great errors. We have spoken of Posidonius as the originator of an error. Dissatisfied with Eratosthenes's measurement of the earth, he had measured it anew. To do this he had observed by a star the arc between Alexandria and Rhodes, and had ascertained the distance as nearly as he could from the number of days' sail between the cities. But soon becoming dissatisfied in regard to the distance he had employed, he adopted Eratosthenes's distance, which the latter had obtained by computing the arc at 700 stadia to