tinguished. When a bullet strikes against a solid
wall, the force of its movement, which seems to disappear, is not lost; it
is converted into heat--the temperature of both the bullet and of that part
of the wall on which it impinges being raised by the concussion. And it is
found that the amount of the heat which is thus produced is always in exact
proportion to the quantity of mechanical motion which is stopped; this
quantity depending on the weight of the bullet, and on the velocity with
which it was moving. And it has been ascertained, moreover, by the most
careful, patient, and many times repeated experiments and calculations,
that the quantity of this heat is exactly the same with that which, through
the medium of steam, or by any other mode of applying it, may be made to
produce the same quantity of mechanical motion that was extinguished in the
bullet. Thus the force was not destroyed, but only converted into another
form.

And if we should follow out the natural effects of this heat into which the
motion of the bullet was transferred, we should find it rarefying the air
around the place of concussion, and thus lifting the whole mass of the
atmosphere above it, and producing currents of the nature of wind, and
through these producing other effects, thus going on forever; the force
changing its form, but neither increasing or diminishing its quantity
through a series of changes without end.

_The Arrest and temporary Reservation of Force_.

Now, although it is thus impossible that any force should be destroyed, or
in any way cease to exist in one form without setting in action a precisely
equal amount in some other form, it may, as it were, pass into a condition
of _restraint_, and remain thus suspended and latent for an indefinite
period--ready, however, to break into action again the moment that the
restraint is removed. Thus a perfectly elastic spring may be bent by a
certain force, and retained in the bent position a long time. But the
moment that it is released it will unbend itself, exercising in so doing
precisely the degree of force expended in bending it. In the same manner
air may be compressed in an air-gun, and held thus, with the force, as it
were, imprisoned, for any length of time, until at last, when the detent is
released by the trigger, the elastic force comes into action, exercising in
its action a power precisely the same as that with which it was compressed.

Force or power may be thus, a