nomon AF. This angle is
equal to the opposite angle JFL, which measures the sun's distance from
the zenith; and which is also equal to the angle AES--to determine the
Size of which is the real object of the entire measurement.

FIG. 2 shows the form of the gnomon actually employed in antiquity. The
hemisphere KA being marked with a scale, it is obvious that in actual
practice Eratosthenes required only to set his gnomon in the sunlight at
the proper moment, and read off the answer to his problem at a glance.
The simplicity of the method makes the result seem all the more
wonderful.}

Of course it is the method, and not its details or its exact results,
that excites our interest. And beyond question the method was an
admirable one. Its result, however, could not have been absolutely
accurate, because, while correct in principle, its data were defective.
In point of fact Syene did not lie precisely on the same meridian as
Alexandria, neither did it lie exactly on the tropic. Here, then,
are two elements of inaccuracy. Moreover, it is doubtful whether
Eratosthenes made allowance, as he should have done, for the
semi-diameter of the sun in measuring the angle of the shadow. But
these are mere details, scarcely worthy of mention from our present
stand-point. What perhaps is deserving of more attention is the fact
that this epoch-making measurement of Eratosthenes may not have been the
first one to be made. A passage of Aristotle records that the size of
the earth was said to be 400,000 stadia. Some commentators have thought
that Aristotle merely referred to the area of the inhabited portion
of the earth and not to the circumference of the earth itself, but his
words seem doubtfully susceptible of this interpretation; and if he
meant, as his words seem to imply, that philosophers of his day had a
tolerably precise idea of the globe, we must assume that this idea was
based upon some sort of measurement. The recorded size, 400,000 stadia,
is a sufficient approximation to the truth to suggest something more
than a mere unsupported guess. Now, since Aristotle died more than fifty
years before Eratosthenes was born, his report as to the alleged size of
the earth certainly has a suggestiveness that cannot be overlooked; but
it arouses speculations without giving an inkling as to their solution.
If Eratosthenes had a precursor as an earth-measurer, no hint or rumor
has come down to us that would enable us to guess who that precurso