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.
Geographical knowledge however was most notably advanced by Strabo, who lived in the Augustan era; although his researches were chiefly confined to the Roman empire. Strabo was, like Herodotus, a great traveller, and much of his geographical information is the result of his own observations. It is probable he was much indebted to Eratosthenes, who preceded him by three centuries. The authorities of Strabo were chiefly Greek, but his work is defective from the imperfect notions which the ancients had of astronomy; so that the determination of the earth's figure by the measure of latitude and longitude, the essential foundation of geographical description, was unknown. The enormous strides which all forms of physical science have made since the discovery of America throw all ancient descriptions and investigations into the shade, and Strabo appears at as great disadvantage as Pliny or Ptolemy; yet the work of Strabo, considering his means, and the imperfect knowledge of the earth's surface and astronomical science in his day, was really a great achievement. He treats of the form and magnitude of the earth, and devotes eight books to Europe, six to Asia, and one to Africa. The description of places belongs to Strabo, whose work was accepted as the text-book of the science till the fifteenth century, for in his day the Roman empire had been well surveyed. He maintained that the earth is spherical, and established the terms longitude and latitude, which Eratosthenes had introduced, and computed the earth to be one hundred and eighty thousand stadia in circumference, and a degree to be five hundred stadia in length, or sixty-two and a-half Roman miles. His estimates of the length of a degree of latitude were nearly correct; but he made great errors in the degrees of longitude, making the length of the world from east to west too great, which led to the belief in the practicability of a western passage to India. He also assigned too great length to the Mediterranean, arising from the difficulty of finding the longitude with accuracy. But it was impossible, with the scientific knowledge of his day, to avoid errors, and we are surprised that he made so few.
Whatever may be said of the accuracy of the great geographer of antiquity, it cannot be denied that he was a man of immense research and learning. His work in seventeen books is one of the most valuable that have come down from antiquity, both from the discussions which run through it, and the curious facts which can be found nowhere else. It is scarcely fair to estimate the genius of Strabo by the correctness and extent of his geographical knowledge. All men are comparatively ignorant in science, because science is confessedly a progressive study. The great scientific lights of our day may be insignificant, compared with those who are to arise, if profundity and accuracy of knowledge be made the test. It is the genius of the ancients, their grasp and power of mind, their original labors, which we are to consider.
Thus it would seem that among the ancients, in those departments of science which are inductive, there were not sufficient facts, well established, from which to make sound inductions; but in those departments which are deductive, like pure mathematics, and which require great reasoning powers, there were lofty attainments,--which indeed gave the foundation for the achievements of modern science.
AUTHORITIES.
An exceedingly learned work (London, 1862) on the Astronomy of the Ancients, by Sir George Cornewall Lewis, though rather ostentatious in the parade of authorities, and minute on points which are not of much consequence, is worth consulting. Delambre's History of Ancient Astronomy has long been a classic, but is richer in materials for a history than a history itself. There is a valuable essay in the Encyclopaedia Britannica, which refers to a list of special authors. Whewell's History of the Inductive Sciences may also be consulted with profit. Dunglison's History of Medicine is a standard, giving much detailed information, and Leclerc among the French and Speugel among the Germans are esteemed authorities. Strabo's Geography is the most valuable of antiquity; see also Polybius: both of these have been translated and edited for English readers.