This led to a truly startling thought. Since the heavenly bodies are of the same composition as the earth, and since they are observed to be whirling about the earth in space, may we not suppose that they were once a part of the earth itself, and that they have been thrown off by the force of a whirling motion? Such was the conclusion which Anaxagoras reached; such his explanation of the origin of the heavenly bodies. It was a marvellous guess. Deduct from it all that recent science has shown to be untrue; bear in mind that the stars are suns, compared with which the earth is a mere speck of dust; recall that the sun is parent, not daughter, of the earth, and despite all these deductions, the cosmogonic guess of Anaxagoras remains, as it seems to us, one of the most marvellous feats of human intelligence. It was the first explanation of the cosmic bodies that could be called, in any sense, an anticipation of what the science of our own day accepts as a true explanation of cosmic origins. Moreover, let us urge again that this was no mere accidental flight of the imagination; it was a scientific induction based on the only data available; perhaps it is not too much to say that it was the only scientific induction which these data would fairly sustain. Of course it is not for a moment to be inferred that Anaxagoras understood, in the modern sense, the character of that whirling 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.