342 PTOLEMY AND LATER GEOGRAPHERS. [CHAP.
world, because he greatly over-estimated this relatively to the
true circumference of the earth. There was, however,' another
cause which contributed even more largely to this error—
namely, the tendency to exaggerate distances on the part of
seamen and traders, on whose reckonings, as we have seen, in
default of astronomical observations, which were few in number,
he was forced to depend It is greatly to Ptolemy's credit
that, whereas all previous geographers had accepted these
without qualification, he clearly perceived the necessity of
making allowance for the deviations from the direct line, and
the varying rate of progress, which were produced by the
windings of roads or the irregular force and direction of winds,
and of reducing the distances on the map accordingly1; his
only fault was that his corrections were made on too limited a
scale, so that a very considerable excess still remained. On the
The other hand, a certain error in defect in his calcula-
Fortunate tion arose from his assuming the Fortunate Isles
Prime Men- (the Canaries) as the point from which his longi-
dian- tudes were to be reckoned. The westernmost island
of this group, Ferro, long continued to be treated as the prime
meridian, and is so among some German geographers at the
present day; but in Ptolemy's time the position of those islands
was not determined, and accordingly it was only by conjecture
that he placed them two degrees and a half to the westward of the
Sacred Promontory (Cape St. Vincent), instead of about nine
degrees, which is the true estimate. The total result which he pro-
duced for the length of the known world, from the Fortunate Isles
in the west to the city of Sera in China towards the east, was 180°,
whereas the reality is about 130°. In one respect this mistake
was advantageous in the consequences which proceeded from it at
a later period, for by diminishing the interval between the eastern
and western extremities of the world it encouraged the idea that
the passage from the one to the other might be accomplished,
and thus indirectly contributed to the discovery of America by
Columbus. Its breadth he estimated at 80°, from the parallel of
Thule (the Shetlands) to that of the Prasum Promontorium
(perhaps Cape Delgado) on the eastern coast of Africa to
1 i. 2. 4.