yet it
was one that could be refuted with the greatest ease by reasoning
from strictly analogous experiments. It might readily be observed, for
example, that a stone dropped from a moving cart does not strike the
ground directly below the point from which it is dropped, but partakes
of the forward motion of the cart. If any one doubt this he has but to
jump from a moving cart to be given a practical demonstration of the
fact that his entire body was in some way influenced by the motion of
translation. Similarly, the simple experiment of tossing a ball from the
deck of a moving ship will convince any one that the ball partakes of
the motion of the ship, so that it can be manipulated precisely as
if the manipulator were standing on the earth. In short, every-day
experience gives us illustrations of what might be called compound
motion, which makes it seem altogether plausible that, if the earth is
in motion, objects at its surface will partake of that motion in a way
that does not interfere with any other movements to which they may
be subjected. As the Copernican 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