The Inscription To Which We Have Alluded Was Extant In The Time
Of Cosmas (A.D. 545), By Whom It Was Seen.
From it, Ptolemy appears to have
passed to the Tacazze, which he calls the Nile, and to have penetrated into
Gojam, in which province the fountains of the Nile are found.
He made
roads, opened a communication between this country and Egypt, and during
this expedition obliged the Arabians to pay tribute, and to maintain the
roads free from robbers and the sea from pirates; subduing the whole coast
from [Leucke->Leuke] Come to Sabea. The inscription adds: "In the
accomplishment of this business I had no example to follow, either of the
ancient kings of Egypt, or of my own family; but was the first to conceive
the design, and to carry it into execution. Thus, having reduced the whole
world to peace under my own authority, I came down to Aduli, and sacrificed
to Jupiter, to Mars, and to Neptune, imploring his protection for all who
navigate these seas."
Ptolemy Euergetes was particularly attentive to the interests of the
library at Alexandria. The first librarian appointed by Ptolemy the
successor of Alexander, was Zenodotus; on his death, Ptolemy Euergetes
invited from Athens Eratosthenes, a citizen of Cyrene, and entrusted to him
the care of the library: it has been supposed that he was the second of
that name, or of an inferior rank in learning and science, because he is
sometimes called Beta; but by this appellation nothing else was meant, but
that he was the second librarian of the royal library at Alexandria. He
died at the age of 81, A.C. 194. He has been called a second Plato, the
cosmographer and the geometer of the world: he is rather an astronomer and
mathematician than a geographer, though geography is indebted to him for
some improvements in its details, and more especially for helping to raise
it to the accuracy and dignity of a science. By means of instruments, which
Ptolemy erected in the museum at Alexandria, he ascertained the obliquity
of the ecliptic to be 23 deg. 51' 20". He is, however, principally celebrated
as the first astronomer who measured a degree of a great circle, and thus
approximated towards the real diameter of the earth.
The importance of this discovery will justify us in entering on some
details respecting the means which this philosopher employed, and the
result which he obtained.
It is uncertain whether the well at Syene, in Upper Egypt, which he used
for this purpose, was dug by his directions, or existed previously. Pliny
seems to be of the former opinion; but there is reason to believe that it
had a much higher antiquity. The following observations on its structure by
Dr. Horsley, Bishop of Rochester, are ingenious and important. "The well,
besides that it was sunk perpendicularly, with the greatest accuracy, was,
I suppose, in shape an exact cylinder. Its breadth must have been moderate,
so that a person, standing upon the brink, might safely stoop enough over
it to bring his eye into the axis of the cylinder, where it would be
perpendicularly over the centre of the circular surface of the water. The
water must have stood at a moderate, height below the mouth of the well,
far enough below the mouth to be sheltered from the action of the wind,
that its surface might be perfectly smooth and motionless; and not so low,
but that the whole of its circular surface might be distinctly seen by the
observer on the brink. A well formed in this manner would afford, as I
apprehend, the most certain observation of the sun's appulse to the zenith,
that could be made with the naked eye; for when the sun's centre was upon
the zenith, his disc would be seen by reflection on the water, in the very
middle of the well, - that is, as a circle perfectly concentric with the
circle of the water; and, I believe, there is nothing of which the naked
eye can judge with so much precision as the concentricity of two circles,
provided the circles be neither very nearly equal, nor the inner circle
very small in proportion to the outer."
Eratosthenes observed, that at the time of the summer solstice this well
was completely illuminated by the sun, and hence he inferred that the sun
was, at that time, in the zenith of this place. His next object was to
ascertain the altitude of the sun, at the same solstice, and on the very
same day, at Alexandria. This he effected by a very simple contrivance: he
employed a concave hemisphere, with a vertical style, equal to the radius
of concavity; and by means of this he ascertained that the arch,
intercepted between the bottom of the style and the extreme point of its
shadow, was 7 deg. 12'. This, of course, indicated the distance of the sun from
the zenith of Alexandria. But 7 deg. 12' is equal to the fiftieth part of a
great circle; and this, therefore, was the measure of the celestial arc
contained between the zeniths of Syene and Alexandria. The measured
distance between these cities being 5000 stadia, it followed, that 5000 X
50 = 250,000, was, according to the observations of Eratosthenes, the
extent of the whole circumference of the earth.
If we knew exactly the length of the stadium of the ancients, or, to speak
more accurately, what stadium is referred to in the accounts which have
been transmitted to us of the result of the operations of Eratosthenes,
(for the ancients employed different stadia,) we should be able precisely
to ascertain the circumference which this philosopher ascribed to the
earth, and also, whether a nearer approximation to the truth was made by
any subsequent or prior ancient philosopher. The circumference of the earth
was conjectured, or ascertained, by Aristotle, Cleomedes, Posidonius, and
Ptolemy respectively, to be 400, 300, 240, and 180 thousand stadia. It is
immediately apparent that these various measures have some relation to each
other, and probably express the same extent; measured in different stadia;
and this probability is greatly increased by comparing the real distances
of several places with the ancient itinerary distances.
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