The Story of Geographical Discovery | Page 8

to the
south, than it really is; but it also contains an account of the more
easterly parts of Asia, Indo-China, and China itself, "where the silk
comes from." This had an important influence on the views of Ptolemy,
as we shall see, and indirectly helped long afterwards to the discovery
of America.
[Illustration: PTOLEMAEI ORBIS]
It was left to PTOLEMY of Alexandria to sum up for the ancient world
all the knowledge that had been accumulating from the time of
Eratosthenes to his own day, which we may fix at about 150 A.D. He
took all the information he could find in the writings of the preceding

four hundred years, and reduced it all to one uniform scale; for it is to
him that we owe the invention of the method and the names of latitude
and longitude. Previous writers had been content to say that the
distance between one point and another was so many stadia, but he
reduced all this rough reckoning to so many degrees of latitude and
longitude, from fixed lines as starting-points. But, unfortunately, all
these reckonings were rough calculations, which are almost invariably
beyond the truth; and Ptolemy, though the greatest of ancient
astronomers, still further distorted his results by assuming that a degree
was 500 stadia, or 50 geographical miles. Thus when he found in any
of his authorities that the distance between one port and another was
500 stadia, he assumed, in the first place, that this was accurate, and, in
the second, that the distance between the two places was equal to a
degree of latitude or longitude, as the case might be. Accordingly he
arrived at the result that the breadth of the habitable globe was, as he
put it, twelve hours of longitude (corresponding to 180°)--nearly
one-third as much again as the real dimensions from Spain to China.
The consequence of this was that the distance from Spain to China
westward was correspondingly diminished by sixty degrees (or nearly
4000 miles), and it was this error that ultimately encouraged Columbus
to attempt his epoch-making voyage.
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 up to the beginning of the last
century, just as his astronomical work was only finally abolished by the
work of Newton. Ptolemy has thus the rare distinction of being the
greatest authority on two important departments of human
knowledge--astronomy and geography--for over fifteen hundred years.
Into the details of his description of the world it is unnecessary to go.
The map will indicate how near he came to the main outlines of the
Mediterranean, of Northwest Europe, of Arabia, and of the Black Sea.
Beyond these regions he could only depend upon the rough indications
and guesses of untutored merchants. But it is worth while referring to
his method of determining latitude, as it was followed up by most
succeeding geographers. Between the equator and the most northerly
point known to him, he divides up the earth into