Eratosthenes, though more properly an astronomer, and the most distinguished among the ancients, was also a considerable writer on geography, indeed, the first who treated the subject systematically, although none of his writings have reached us. The improvements he pointed out were applied by Ptolemy himself. His work was a presentation of the geographical knowledge known in his day, so far as geography is the science of determining the position of places on the earth's surface. When Eratosthenes began his labors, in the third century before Christ, it was known that the surface of the earth was spherical; he established parallels of latitude and longitude, and attempted the difficult undertaking of measuring the circumference of the globe by the actual measurement of a segment of one of its great circles. Hipparchus (beginning of second century before Christ) introduced into geography a great improvement; namely, the relative situation of places, by the same process that he determined the positions of the heavenly bodies. He also pointed out how longitude might be determined by observing the eclipses of the sun and moon. This led to the construction of maps; but none have reached us except those that were used to illustrate the geography of Ptolemy. Hipparchus was the first who raised geography to the rank of a science. He starved himself to death, being tired of life. Posidonius, who was nearly a century later, determined the arc of a meridian between Rhodes and Alexandria to be a forty-eighth part of the whole circumference,--an enormous calculation, yet a remarkable one in the infancy of astronomical science. His writings on history and geography are preserved only in quotations by Cicero, Strabo, and others.