ment of that profound fundamental physical principle, the conservation of energy. Even before the beginning of our half-century, Davy and Rumford (especially the latter) had caught faint glimpses of the coming truth in this direction. They recognized that heat was a mode of motion, and Rumford went so far as to observe that the energy generated by a given amount of hay burned in an engine might be measured against the energy generated by the same amount of hay consumed by horses. But to Dr. Joule, of Manchester, in our own time, is due the first great onward movement, in the discovery and determination of the mechanical equivalent of heat. Joule's numerous experiments on the exact relation between heat and mechanical energy resulted in the establishment of a formula of equivalence in terms of kilogrammetres necessary to raise by one degree centigrade the temperature of one kilogramme of water. More properly put, he showed that the energy required to raise a weight of one hundred pounds through one foot was equivalent to the amount required to raise a certain fixed quantity of water through one degree in temperature.
Starting from this settled point, it soon became clear to physical thinkers that every species of energy was more or less readily convertible into every other, and that an exact numerical equivalence existed between them. This principle, which first clearly emerged into the consciousness of physicists about the middle decades of the present century, was originally known under the name of "Persistence of Force," in which form Grove's well-known little treatise helped largely to popularize its acceptance. But, as time went on, the underlying distinction between force and energy came to be more definitely realized, and the phrase conservation of energy began to supersede the older and erroneous terminology. The realization of the varying nature of energy as potential and kinetic helped in the transformation of the prime concept. At last, under the hands of Clausius, Helmholtz, Mayer, Clerk Maxwell, Tait, and Balfour Stewart, the doctrine assumed its modern form—that all energies are mutually convertible, and that the sum-total of energy, potential and kinetic, is a constant quantity throughout the cosmos.
The practical applications of the doctrine of energy are as yet only in their infancy. The whole mass of theoretical science has to be re-written in accordance with this new and fundamental law. The whole field of applied science has to be developed and enlarged by the light of this pregnant and universal principle. Its implications are all-pervading. In astronomy it has profoundly affected all our conceptions as to the sun's heat, the orbits of planets, the nature of meteors, the past, present, and future of the universe. In biology it has taught us to envisage the plant mainly as a machine in which kinetic energy is being transformed into potential; the animal mainly as a machine in which potential energy is being transformed back again into kinetic. In mechanics and the mechanical arts it has produced
