Ptolemy's errors of calculation would not have been so extensive
but that he adopted a method of measurement which made them
accumulative. If he had chosen Alexandria for the point of departure
in measuring longitude, the errors he made when reckoning westward
would have been counterbalanced by those reckoning eastward, and
would not have resulted in any serious distortion of the truth; but
instead of this, he adopted as his point of departure the Fortunatæ
Insulæ, or Canary Islands, and every degree measured to the east
of these was one-fifth too great, since he assumed that it was
only fifty miles in length. I may mention that so great has been
the influence of Ptolemy on geography, that, up to the middle of
the last century, Ferro, in the Canary Islands, was still retained
as the zero-point of the meridians of longitude.

Another point in which Ptolemy's system strongly influenced modern
opinion was his departure from the previous assumption that the
world was surrounded by the ocean, derived from Homer. Instead
of Africa being thus cut through the middle by the ocean, Ptolemy
assumed, possibly from vague traditional knowledge, that Africa
extended an unknown length to the south, and joined on to an equally
unknown continent far to the east, which, in the Latinised versions
of his astronomical work, was termed "terra australis incognita,"
or "the unknown south land." As, by his error with regard to the
breadth of the earth, Ptolemy led to Columbus; so, by his mistaken
notions as to the "great south land," he prepared the way for the
discoveries of Captain Cook. But notwithstanding these errors,
which were due partly to the roughness of the materials which he
had to deal with, and partly to scientific caution, Ptolemy's work
is one of the great monuments of human industry and knowledge. For
the Old World it remained the basis of all geographical knowledge