8
feet, would not be an inordinate one for a cannon-ball as it quits the
gun. Hence, a cannon-ball moving with a velocity of 1,338 feet a
second, would, by collision, generate an amount of heat competent to
raise its own weight of water 36 degrees Fahrenheit in temperature. If
composed of iron, and if all the heat generated were concentrated in
the ball itself, its temperature would be raised about 360 degrees
Fahrenheit; because one degree in the case of water is equivalent to
about ten degrees in the case of iron. In artillery practice, the
heat generated is usually concentrated upon the front of the bolt, and
on the portion of the target first struck. By this concentration the
heat developed becomes sufficiently intense to raise the dust of the
metal to incandescence, a flash of light often accompanying collision
with the target.

Let us now fix our attention for a moment on the gunpowder which urges
the cannon-ball. This is composed of combustible matter, which if
burnt in the open air would yield a certain amount of heat. It will
not yield this amount if it perform the work of urging a ball. The
heat then generated by the gunpowder will fall short of that produced
in the open air, by an amount equivalent to the _vis viva_ of the ball;
and this exact amount is restored by the ball on its collision with
the target. In this perfect way are heat and mechanical motion
connected.

Broadly enunciated, the principle of the conservation of force
asserts, that the quantity of force in the universe is as unalterable
as the quantity of matter; that it is alike impossible to create force
and to annihilate it. But in what sense are we to understand this
assertion? It would be manifestly inapplicable to the force of
gravity as defined by Newton; for this is a force varying inversely as
the square of the distance; and to affirm the constancy of a varying
force would be self-contradictory. Yet, when the question is properly
understood, gravity forms no exception to the law of conservation.
Following the method pursued by Helmholtz, I will here attempt an
elementary exposition of this law. Though destined in its
applications to produce momentous changes in human thought, it is not
difficult of comprehension.

For the sake of simplicity we will consider a particle of matter,
which we may call F, to be perfectly fixed, and a second movable
particle, D, placed at a distance from F. We will assume that these
two particl