IX.] MERIDIANS OF LONGITUDE. 177
mathematical geography, and restricted it to a narrow circle of
students1.
The attempt to determine meridians of longitude was attended
with far greater difficulties than any which stood in
the way of measurements of latitude. In default
of the magnetic needle, there was no instrument in
this case which could afford help, as the gnomon did for parallels;
and moreover, since the ancients divided the day and the night
into twelve hours each, irrespectively of the difference of the two
at different times of the year, the length of the hours varied,
except at the equinoxes, so that comparisons of the time of day
at different places could not fail to be inexact. Consequently,
the estimate in every instance was made to depend entirely on the
calculations of unscientific observers. In treating of Herodotus,
we have noticed how that writer feels his way towards a meridian
line, when he describes the situation of Pteria by its position
relatively to Sinope, which lay due north of it j and also when, in
tracing the correspondence between the mouths of the Nile and
the Ister, which he supposed to be opposite to one another, he
draws a line between them through Cilicia Tracheia and Sinope2.
This line was the best that was available for the Greeks for the
purpose of observing a meridian, because their traders reached
both ends of it in the course of their communication with Egypt
on the one hand and their colonies to the north of the Euxine on
the other. In consequence of this, and also, after the foundation
of Alexandria, of the facilities which existed for pursuing the same
line to the southward along the Nile valley, this direction was
followed by the meridian which, as we have seen, was chosen for
the earliest measurement of the earth's circumference, and was
subsequently adopted by Eratosthenes. The points through which
the last-named geographer drew it were, to the southward of
Alexandria, Syene and Meroe, and to the northward, Rhodes,
1 The detailed account of the different climata of the inhabited world,
which Strabo has given in 2. 5. 35 foil., is irreconcilable with his statement of
Hipparchus' views in § 34, and must be regarded in the main as emanating
from Strabo himself, and not from Hipparchus. See Berger, Die geographischen
Fragment* des Hipparch, pp. 41, 42.
a supra, pp. 79, So.
T. 12