The ancients, as well as the moderns, define _motion_ to be change of
place, or the passing from one place to another; they knew the
acceleration of bodies in falling, but not so exactly as to determine
its law or cause. It was an axiom of Aristotle and the Peripatetics,
that a body in falling acquired a celerity of motion, proportionable to
its distance from the place whence the motion began; but they knew not
that this increase of the celerity of falling bodies was uniform, and
that the spaces passed over in equal times increased proportionably to
the unequal numbers 1, 3, 5, 7, &c. Two mistakes of Aristotle hindered
him from arriving at the truth. The first was, that there were two
tendencies in body; one downwards, carrying it to the centre, in those
that were heavy; the other upwards, removing it from it, in those that
were light. His second error was, that he thought different bodies
rolled through space with a celerity proportional to their masses. He
did not consider that the resistance of the medium was the only cause of
this difference; for supposing them to move through an irresisting
medium, or in _vacuo_, the lightest bodies would then fall with the same
velocity as the heaviest. This is demonstrated by means of the air-pump,
wherein paper, lead, and gold, descend with equal swiftness.
Yet all the ancients were not thus ignorant. Lucretius, instructed in
the principles of Democritus and Epicurus, arrived at this knowledge,
and supports it by such arguments, as might do honour to the most
experienced naturalist of our times.--“Admitting that there was nothing
in the vacuum to resist the motion of bodies, it necessarily followed,
that the lightest would descend with a celerity equal to the weightiest;
that where there was no resistance in the medium, bodies must always
move through equal spaces in equal times; but that the case would be
different in such mediums, as opposed divers degrees of resistance to
the bodies passing through them.” Hereupon, he alleges the very same
reasonings which Galileo draws from experience to support his theory. He
says, that “the difference of velocities ought to increase or abate,
according to the difference of resistance in the medium; and that
because air and water resist bodies differently, they fall through these
mediums with different degrees of velocity.” We shall presently see,
that the ancients were acquainted with the principle of _gravitation_.
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