g force which we call
centrifugal. About two thousand years were yet to elapse before that
force was explained as elementary inertia; and even that explanation,
let us not forget, merely sufficed to push back the barriers of mystery
by one other stage; for even in our day inertia is a statement of fact
rather than an explanation.

But however little Anaxagoras could explain the centrifugal force
on mechanical principles, the practical powers of that force were
sufficiently open to his observation. The mere experiment of throwing
a stone from a sling would, to an observing mind, be full of
suggestiveness. It would be obvious that by whirling the sling about,
the stone which it held would be sustained in its circling path about
the hand in seeming defiance of the earth's pull, and after the stone
had left the sling, it could fly away from the earth to a distance which
the most casual observation would prove to be proportionate to the speed
of its flight. Extremely rapid motion, then, might project bodies from
the earth's surface off into space; a sufficiently rapid whirl would
keep them there. Anaxagoras conceived that this was precisely what
had occurred. His imagination even carried him a step farther--to a
conception of a slackening of speed, through which the heavenly bodies
would lose their centrifugal force, and, responding to the perpetual
pull of gravitation, would fall back to the earth, just as the great
stone at aegespotomi had been observed to do.

Here we would seem to have a clear conception of the idea of universal
gravitation, and Anaxagoras stands before us as the anticipator of
Newton. Were it not for one scientific maxim, we might exalt the old
Greek above the greatest of modern natural philosophers; but that maxim
bids us pause. It is phrased thus, "He discovers who proves." Anaxagoras
could not prove; his argument was at best suggestive, not demonstrative.
He did not even know the laws which govern falling bodies; much less
could he apply such laws, even had he known them, to sidereal bodies at
whose size and distance he could only guess in the vaguest terms. Still
his cosmogonic speculation remains as perhaps the most remarkable one of
antiquity. How widely his speculation found currency among his immediate
successors is instanced in a passage from Plato, where Socrates is
represented as scornfully answering a calumniator in these terms: "He
asserts that I say the sun is a stone and the moon an eart