IX.] SPHERICAL FORM OF THE EARTH. 167
first demands our attention, because on it almost every point
connected with mathematical geography depends. .
The Pythagoreans were the earliest teachers who Form of the
maintained this doctrine, but, as far as we can
ascertain, they did so, not by means of any formal proof, but
on the ground of the fitness of things, because the circle is the
most perfect figure. This view met with no acceptance from the
philosophers of the Ionian school, nor was it adopted by Heca-
taeus, or even by Herodotus1. It is probable enough that
Eudoxus furnished mathematical proof of it, for he was well
qualified to do so by the knowledge of astronomy which he pos-
sessed, but on this point we have no evidence. The first writer
in whose works definite arguments on the subject are found is
Aristotle, His mode of proof is twofold. First he . ,
deduces it from the law of gravitation, or, as he Arguments
expresses it, the tendency of all things towards the
centre. By the action of this, when the earth was in the course
of formation, and the component elements were coming together
equally from every quarter, the mass thus formed by accretion
was so constituted that its entire circumference must be equi-
distant from its centre. Secondly, he infers it from what is seen
to take place in lunar eclipses; for, when the earth is inter-
posed between the sun and the moon, the spherical form of the
obscured part of the moon's surface shews that the body which
causes the obscuration is also spherical9. It appears strange
that the proof which to us is the most familiar, be-
cause it appeals directly to the eye and to ordinary
experience, viz. the sight of distant objects gradu-
ally revealing themselves above the horizon:—
*the first beam glittering on a sail,
That brings our friends up from the under world*—
should not have been employed until a late period by the ancients.
We can hardly think it did not occur to them; and by a converse
line of argument Archimedes (250 B.C.), who regarded the spheri-
city of the earth as sufficiently proved, deduced from that doctrine
1 v. sufra, pp. 60, 7«, 78.
* Aristot. De Caelo, a. 14. 8, 9, 13.