His Geography, Such As We Have Described It, Consists Of Eight Books, And
Is Certainly Much More Scientific Than Any Which Had Been Previously
Written On This Science.
In it there appears, for the first time, an
application of geometrical principles to the construction of maps:
The
different projections of the sphere, and a distribution of the several
places on the earth, according to their latitude and longitude. Geography
was thus established on its proper principles, and intimately connected
with astronomical observations and mathematical science. The utility and
merit of Ptolemy's work seems to have been understood and acknowledged soon
after it appeared. Agathemidorus, who lived not long after him, praises him
for having reduced geography to a regular system; and adds, that he treats
of every thing relating to it, not carelessly, or merely according to the
ideas of his own, but to what had been delivered by more ancient authors,
adopting from them whatever he found consonant to truth. Agathodaemon, an
artist of Alexandria, observing the request in which his work was held,
prepared a set of maps to illustrate it, in which all the places mentioned
in it were laid down, with the latitudes and longitudes he assigned them.
The reputation of his geography remained unshaken and undiminished during
the middle ages, both in Arabia and Europe; and even now, the scientific
language which he first employed, is constantly used, and the position of
places ascertained by specifying their latitude and longitude.
It was not to be expected, however, that Ptolemy could accurately fix the
longitude and latitude of places in the remoter parts of the then known
world; his latitudes and longitudes are accordingly frequently erroneous,
but especially the latter. This arose partly from his taking five hundred
stadia for a degree of a great circle, and partly from the vague method of
calculating distances, by the estimate of travellers and merchants, and the
number of days employed in their journies by land, and voyages by sea. As
he took seven hundred stadia for a degree of latitude, his errors in
latitude are not so important; and though the latitude he assigns to
particular places is incorrect, yet the length of the globe, according to
him, or the distance from the extreme points north and south, then known,
is not far from the truth. Thus the latitude of Thule, according to
Ptolemy, is 64 degrees north, and the parallel through the cinnamon country
16 deg. 24' south, that is, 80 deg. 24' on the whole, a difference from the truth
of not more than six or seven degrees. It is remarked by D'Anville, and Dr.
Vincent coincides in the justice of the remark, that the grandest mistake
in the geography of Ptolemy has led to the greatest discovery of modern
times. Strabo had affirmed, that nothing obstructed the passage from Spain
to India by a westerly course, but the immensity of the Atlantic ocean;
but, according to Ptolemy's errors in longitude, this ocean was lessened by
sixty degrees; and as all the Portuguese navigators were acquainted with
his work, as soon as it was resolved to attempt a passage to India, the
difficulty was, in their idea, lessened by sixty degrees; and when Columbus
sailed from Spain, he calculated on sixty degrees less than the real
distance from that country to India.
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