Plato, who was familiar with the doctrines of the Pythagoreans, adopted their view of the form of the earth, and did much to popularize it among his countrymen.[261] To the generation that succeeded him, the sphericity of the earth was a fact as capable of logical demonstration as a geometrical theorem. Aristotle, in his treatise “On the Heaven,” after detailing the views of those philosophers who regarded the earth as flat, drum-shaped, or cylindrical, gives a formal summary of the grounds which necessitate the assumption of its sphericity, specifying the tendency of all things to seek the centre, the unvarying circularity of the earth’s shadow at eclipses of the moon, and the proportionate change in the altitude of stars resulting from changes in the observer’s latitude. Aristotle made the doctrine orthodox; his successors, Eratosthenes, Hipparchus, and Ptolemy, constituted it an inalienable possession of the race. Greece transmitted it to Rome, Rome impressed it upon barbaric Europe; taught by Pliny, Hyginus, Manilius, expressed in the works of Cicero, Virgil, Ovid, it passed into the school-books of the Middle Ages, whence, reinforced by Arabian lore, it has come down to us.[262] That the belief ever became in antiquity or in the Middle Ages widely spread among the people is improbable; it did not indeed escape opposition among the educated; writers even of the Augustan age sometimes appear in doubt.[263] The sphericity of the earth once comprehended, there follow certain corollaries which the Greeks were not slow to perceive. Plato, indeed, who likened the earth to a ball covered with party-colored strips of leather, gives no estimate of its size, although the description of the world in the _Phaedo_ seems to imply immense magnitude;[264] but Aristotle states that mathematicians of his day estimated the circumference at 400,000 stadia,[265] and Archimedes puts the common reckoning at somewhat less than 300,000 stadia.[266] How these figures were obtained we are not informed. The first measurement of the earth which rests on a known method was that made about the middle of the third century B.C., by Eratosthenes, the librarian at Alexandria, who, by comparing the estimated linear distance between Syene, under the tropic, and Alexandria with their angular distance, as deduced from observations on the shadow of the gnomon at Alexandria, concluded that the circumference of the earth was 250,000 or 252,000 stadia.[267] This result, owing to an uncertainty as to the exact length of the stade used in the computation, cannot be interpreted with confidence, but if we assume that it was in truth about twelve per cent. too large, we shall probably not be far out of the way.[268] Hipparchus, in many matters the opponent of Eratosthenes, adopted his conclusion on this point, and was followed by Strabo,[269] by Pliny, who regarded the attempt as somewhat over-bold, but so cleverly argued that it could not be disregarded,[270] and by many others.