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.
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