When a moving body is stopped, its force is not annihilated, but simply takes another form. When the sledge-hammer strikes the leaden bullet and comes to rest, the mechanical force is not destroyed, but is simply converted into heat; and if all the heat produced could be collected, it would be exactly sufficient, when reconverted into mechanical force, to raise the hammer again to the height from which it fell. So, when bodies are rubbed together, their surface-particles are brought into collision, 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."