Page:Popular Science Monthly Volume 16.djvu/253

, the first Greek to give the angle of the ecliptic—as subtended by the side of a pentadecagon, or equal to 24°—and Eudoxus, who wrote a work on the "Period or Circumference of the Earth."

This brings us down to the school of Alexandria. Alexander had founded near the Canopic mouth of the Nile a city which was destined long to perpetuate his name and glory. The glory was to come not so much from its commercial importance, though it rose to be the chief commercial city of the world, as from its intellectual supremacy. It was the ambition of the Ptolemies to make their capital the intellectual center of the world, and in this they were successful. The Attic and Ionian scholars gave place to the Alexandrian, not only in the department of letters, but also in the domain of science. One of the librarians of the great Alexandrian Library was Eratosthenes ( 270), who may justly be called the Father of Geography. His work is in great part preserved to us in the pages of Strabo and Pliny. Having under his eye everything that had ever been written upon the subject, he first combined the whole into a complete system, which can to-day be restored. The map of the world which he prepared, if less perfect in some respects than Ptolemy's, was in other respects far more perfect; indeed, was the most correct which the world was to see down to the sixteenth century Besides other and doubtless very important data, of which we have no information, Eratosthenes must have had a record of an expedition undertaken in the fifth century  by Hanno, under the direction of the Carthaginian Senate, in which he sailed down the west coast of Africa as far as to the Gulf of Benin; as well as an account of the observations made by the followers of Alexander during his march through Asia, and by his naval commander Nearchus, who conducted the fleet from the mouths of the Indus along the coast to the Euphrates.

After him came Hipparchus, who lived at Rhodes ( 160-145). His great merit was in his use of astronomical observations to determine positions upon the earth, instead of depending upon itinerary distances from a few known points, as had been the method of Eratosthenes, But the age did not appreciate his work, and the science was not to realize the advance which was thus made possible; nearly three centuries must elapse before the fruit of his labors was to appear. Eratosthenes had been able to determine latitude by the heavenly bodies, but not longitude. Hipparchus showed how this also could be done, by observing the eclipses of the sun and moon. Again, he invented the method of projection in map-making, another most valuable contribution to the science which was to be despised until a coming age.

Posidonius is a name to be remembered for an error which he introduced into the science, so successfully that it remained for many centuries. What it was we shall see under Ptolemy.

Strabo (54 ), notwithstanding his voluminous work on