During the past thirty years there has come into common use another term, unknown in any technical sense before that time, namely, _energy_. What was once called the conservation of force is now called the conservation of energy, and we now often hear of forms of energy. Thus, heat is said to be a form of energy, and the forms of energy are convertible into one another, as the so-called forces were formerly supposed to be transformable into one another. We are asked to consider gravitative energy, heat energy, mechanical energy, chemical energy, and electrical energy. When we inquire what is meant by energy, we are informed that it means ability to do work, and that work is measurable as a pressure into a distance, and is specified as foot-pounds. A mass of matter moves because energy has been spent upon it, and has acquired energy equal to the work done on it, and this is believed to hold true, no matter what the kind of energy was that moved it. If a body moves, it moves because another body has exerted pressure upon it, and its energy is called _kinetic energy_; but a body may be subject to pressure and not move appreciably, and then the body is said to possess potential energy. Thus, a bent spring and a raised weight are said to possess potential energy. In either case, _an energized body receives its energy by pressure, and has ability to produce pressure on another body_. Whether or not it does work on another body depends on the rigidity of the body it acts upon. In any case, it is simply a mechanical action--body A pushes upon body B (Fig. 1). There is no need to assume anything more mysterious than mechanical action. Whether body B moves this way or that depends upon the direction of the push, the point of its application. Whether the body be a mass as large as the earth or as small as a molecule, makes no difference in that particular. Suppose, then, that _a_ (Fig. 2) spends its energy on _b_, _b_ on _c_, _c_ on _d_, and so on. The energy of _a_ gives translatory motion to _b_, _b_ sets _c_ vibrating, and _c_ makes _d_ spin on some axis. Each of these has had energy spent on it, and each has some form of energy different from the other, but no new factor has been introduced between _a_ and _d_, and the only factor that has gone from _a_ to _d_ has been motion--motion that has had its direction and quality changed, but not its nature. If we agree that energy is neither created nor annihilated, by any physical process, and if we assume that _a_ gave to _b_ all its energy, that is, all its motion; that _b_ likewise gave its all to _c_, and so on; then the succession of phenomena from _a_ to _d_ has been simply the transference of a definite amount of motion, and therefore of energy, from the one to the other; for _motion has been the only variable factor_. If, furthermore, we should agree to call the translatory motion [alpha], the vibratory motion [beta], the rotary [gamma], then we should have had a conversion of [alpha] into [beta], of [beta] into [gamma]. If we should consider the amount of transfer motion instead of the kind of motion, we should have to say that the [alpha] energy had been transformed into [beta] and the [beta] into [gamma].