192 PHYSICAL GEOGRAPHY. [CHAP
earth, which, as we have seen, had already been determined by
Eratosthenes. In this conclusion of his predecessor Posidonius
was indisposed to acquiesce, and he endeavoured to solve
the problem by means of observations of the position of the
star Canopus at Alexandria and at Rhodes; but, owing to the
inaccuracy both of these measurements and of the computation
of the distance between the two places, he greatly underestimated
the circuit of the globe, reducing it from 250,000 stadia, as
computed by Eratosthenes, to 180,000J. Unfortunately, in con-
sequence of the great weight that was attached to his authority,
this estimate was adopted by his successors, and was even ac-
cepted by Ptolemy.
The history of the discovery by the Greeks of the movement
of the tides is peculiarly interesting, because the
knowledge of this phenomenon was only gradually
obtainable by them, since the Mediterranean is, except in a slight
degree, and in certain limited areas, such as the head of the
Adriatic, a tideless sea. Herodotus notices the ebb and flow of
the water in the Maliac gulf8, and also in the Red Sea3; and he
uses the same terms in which these are described, when speaking
of the extraordinary reflux and flux of the waves, which caused
the destruction of a portion of the Persian force under the
command of Artabazus which was besieging Potidaea4. The
shifting currents of the Euripus, which served as a starting-point
for speculations on this subject, are noticed by all three of the
Greek tragedians; Aeschylus describing them as the "tides of
Aulis surging to and fro8," and Sophocles as the "groaning
strait6," while Euripides speaks of the " eddies of the whirling
Euripus7." Aristotle, as we have seen, observed
its movements, and he remarks generally on the
tendency of the sea to sway to and fro, when a
1 Strabo, 2. 2. 2.
a Herod., 7.198; $e is T> Mi/XiSa ?rapa K6\irov 6a\d<r<rns, kv r$ a/wrwrfr
re Kal tfifxh Ayd Taffw iiitiprp ylverou,. * Ibid^ 2. n.
* /&«£, 8. 129. 8 Aesch., Ag.9 191; iraXipp6x0ois & AtfXWos T67row.
6 Soph., Ani., 1145; <rrw6&>T0, ?rop0jK6?.
7 Eurip., Iph* Taur.i 6, 7;
djLt# Sbais, a* 0<£/*' Efywr
atipcus i\ta-ffw Kvavtav aXa trr^fi