With regard to the circumnavigability of Africa two opposite opinions were maintained by the ancient Greek and Latin writers whose authority the men of the Middle Ages were wont to quote as decisive of every vexed question. The old Homeric notion of an ocean encompassing the terrestrial world, although mentioned with doubt by Herodotus,[339] continued to survive after the globular form of the earth had come to be generally maintained by ancient geographers. The greatest of these geographers, Eratosthenes, correctly assumed that the Indian ocean was continuous with the Atlantic,[340] and that Africa could be circumnavigated, just as he incorrectly assumed that the Caspian sea was a huge gulf communicating with a northern ocean, by which it would be possible to sail around the continent of Asia as he imagined it.[341] A similar opinion as to Africa was held by Posidonius and by Strabo.[342] It was called in question, however, by Polybius,[343] and was flatly denied by the great astronomer Hipparchus, who thought that certain observations on the tides, reported by Seleucus of Babylon, proved that there could be no connection between the Atlantic and Indian oceans.[344] Claudius Ptolemy, writing in the second century after Christ, followed the opinion of Hipparchus, and carried to an extreme the reaction against Eratosthenes. By Ptolemy's time the Caspian had been proved to be an inland sea, and it was evident that Asia extended much farther to the north and east than had once been supposed. This seems to have discredited in his mind the whole conception of outside oceans, and he not only gave an indefinite northward and eastward extension to Asia and an indefinite southern extension to Africa, but brought these two continents together far to the southeast, thus making the Indian ocean a land-locked sea.[345] [Footnote 339: [Greek: Ton de Okeanon logo men legousi ap' heliou anatoleon arxamenon gen peri pasan rheein, ergo de ouk apodeiknysi.] Herodotus, iv. 8.] [Footnote 340: [Greek: Kai gar kat' auton Eratosthene ten ektos thalattan hapasan syrroun einai, hoste kai ten Hesperion kai ten Erythran thalattan mian einai.] Strabo, i. 3, Sec. 13.] [Footnote 341: Bunbury, _History of Ancient Geography_, vol. i. p. 644.] [Footnote 342: Strabo, ii. 3, Sec. 4; xvii. 3, Sec. 1.] [Footnote 343: [Greek: Kathaper de kai tes Asias kai tes Libyes, katho synaptousin allelais peri ten Aithiopian, oudeis echei legein atrekos heos ton kath' hemas kairon, poteron epeiros esti kata to syneches ta pros ten mesembrian, e thalatte periechetai.] Polybius, iii. 38.] [Footnote 344: Bunbury, _op. cit._ vol. ii. p. 15.] [Footnote 345: See the map of Ptolemy's world, above, p. 264.] [Sidenote: Story of the Phoenician voyage, in the time of Necho.]