doctrine made its way, this idea of compound motion naturally
received more and more attention, and such experiments as those
of Galileo prepared the way for a new interpretation of the
mechanical principles involved.

The great difficulty was that the subject of moving bodies had
all along been contemplated from a wrong point of view. Since
force must be applied to an object to put it in motion, it was
perhaps not unnaturally assumed that similar force must continue
to be applied to keep the object in motion. When, for example, a
stone is thrown from the hand, the direct force applied
necessarily ceases as soon as the projectile leaves the hand. The
stone, nevertheless, flies on for a certain distance and then
falls to the ground. How is this flight of the stone to be
explained? The ancient philosophers puzzled more than a little
over this problem, and the Aristotelians reached the conclusion
that the motion of the hand had imparted a propulsive motion to
the air, and that this propulsive motion was transmitted to the
stone, pushing it on. Just how the air took on this propulsive
property was not explained, and the vagueness of thought that
characterized the time did not demand an explanation. Possibly
the dying away of ripples in water may have furnished, by
analogy, an explanation of the gradual dying out of the impulse
which propels the stone.

All of this was, of course, an unfortunate maladjustment of the
point of view. As every one nowadays knows, the air retards the
progress of the stone, enabling the pull of gravitation to drag
it to the earth earlier than it otherwise could. Were the
resistance of the air and the pull of gravitation removed, the