ision,
mechanical force is destroyed, and heat appears,--the heat of friction.
The conversion of heat into mechanical motion, and of that motion back
again into heat, may be familiarly illustrated in the case of a
railway-train. The heat generated by combustion in the locomotive is
converted into motion of the cars. But when it is desired to stop the
train, what is to be done? Its mechanical force cannot be annihilated;
it can only be transmuted; and so the brakes are applied, and the train
brought to rest by reconverting its motion into heat, as is manifested
by the smoke and sparks produced by the friction. Now, as heat produces
mechanical motion, and mechanical motion heat, they must clearly have
some common quality. The dynamical theory asserts, that, as they are
both modes of motion, they must be mutually and easily convertible. When
a moving mass is checked or stopped, its force is not annihilated, but
the gross, palpable motion is infinitely subdivided and communicated to
the atoms of the body, producing increased vibrations, which appear as
heat. Heat is thus inferred to be, not a material fluid, but a motion
among the ultimate atoms of matter.

The acceptance of this view led to the highly important inquiry, What is
the equivalent relation between mechanical force and heat? or, how much
heat is produced by a definite quantity of mechanical force? To Dr.
Joule, of Manchester, England, is due the honor of having answered this
question, and experimentally established the numerical relation. He
demonstrated that a one-pound weight, falling through seven hundred and
seventy-two feet and then arrested, produces sufficient heat to raise
one pound of water one degree. Hence this is known as the mechanical
equivalent of heat, or "Joule's Law."

The establishment of the principle of correlation between mechanical
force and heat constitutes one of the most important events in the
progress of science. It teaches us that the movements we see around us
are not spontaneous or independent occurrences, but links in the eternal
chain of forces,--that, when bodies are put in motion, it is at the
expense of some previously existing energy, and that, when they come to
rest, their force is not destroyed, but lives on in other forms. Every
motion we see has its thermal value; and when it ceases, its equivalent
of heat is an invariable result. When a cannon-ball strikes the side of
an iron-plated ship, a flash of light shows that col