mechanics--laws a knowledge of which was absolutely essential to
the progress of physical science. The belief in the rotation of
the earth made necessary a clear conception that all bodies at
the surface of the earth partake of that motion quite
independently of their various observed motions in relation to
one another. This idea was hard to grasp, as an oft-repeated
argument shows. It was asserted again and again that, if the
earth rotates, a stone dropped from the top of a tower could not
fall at the foot of the tower, since the earth's motion would
sweep the tower far away from its original position while the
stone is in transit.

This was one of the stock arguments against the earth's motion,
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