moves from the cannon's mouth to the
spot where it rests. But if there is no power in the ball, why does not
the ball of cork discharged from the same gun with the same momentum,
travel to the same distance, at the same rate? The action commences in
both cases with the same projectile force, the same exterior _means_ are
employed, but the results are widely different. The cause of this
difference must be sought for in the comparative power of each substance
to _continue its own movements_.

Every boy who has played at ball has observed these principles. He
throws his ball, which, if not _counteracted_, will continue in a
straight line, _ad infinitum_--without end. But the air impedes its
progress, and gravitation brings it to the ground. When he throws it
against a hard substance, its velocity is not only overcome, but it is
sent back with great force. But if he takes a ball of wax, of snow, or
any strong adhesive substance, it will not bound. How shall we account
to him for this difference? He did the same with both balls. The impetus
given the one was as great as the other, and the resistance of the
intervening substance was as great in one case as the other; and yet,
one bounds and rebounds, while the other sticks fast as a friend, to the
first object it meets. The cause of this difference is to be sought for
in the different capabilities of the respective balls. One possesses a
strong elastic and repelling power; in the other, the attraction of
cohesion is predominant.

Take another example. Let two substances of equal size and form, the one
made of lead, the other of cork, be put upon the surface of a cistern of
water. The external circumstances are the same, but the effects are
widely different--one sinks, the other floats. We must look for the
cause of this difference, not in the opposite qualities of surrounding
matter, but in the things themselves. If you add to the cork another
quality possessed by the lead, and give it the same form, size, and
_weight_, it will as readily sink to the bottom. But this last property
is possessed in different degrees by the two bodies, and hence, while
the one floats upon the water, the other displaces its particles and
sinks to the bottom. You may take another substance; say the mountain
ebony, which is heavier than water, but lighter than lead, and immerse
it in the water; it will not sink with the rapidity of lead, because its
inherent _power_ is not so strong.

Take still