. We have now to consider at some length how grave
was the problem that came up for immediate solution.

* * * * *

[Sidenote: Views of Eratosthenes, B. C. 276-196.]

[Sidenote: Opposing theory of Ptolemy, cir. A. D. 150.]

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 ei