bow and a wound-up spring possess energy of position, because of the separation of their molecules, which we may avail ourselves of by allowing them to fall back into their natural positions.
If a body be allowed to fall, it necessarily loses the energy which it had in virtue of its position as it approaches the ground, but at the same rate it gains a different kind proportional in amount to the space passed through, and consequently to the square of the velocity. In the case of a projectile, or pendulum-bob, there is a gradual transformation from kinetic to potential energy during the ascent, and a retransformation during the descent. If we spend work in raising a weight, bending a bow, or winding up a spring, that work is spent in laying up stores of energy which we may avail ourselves of at any future time. But if in performing any of these actions we encounter resistance, as in the case of friction, part of the work is spent in overcoming it; and, when we endeavor to get back the energy we put forth, we find that we fail by as much as was spent in overcoming the friction. The energy so spent was long a puzzle to scientific minds, and was believed to be absolutely destroyed, until the experiments of Rumford and Davy fully demonstrated that the work spent in overcoming friction was transformed into heat—a form of kinetic energy, it is true, but of such an inferior class as to have entirely escaped the notice of the shrewdest observers. It is, in general, a very easy matter to transform the whole of the potential or kinetic energy of a body into heat; but it becomes quite a different undertaking when we propose to reconvert the heat into either of the other forms.
Carnot was the first who made any progress in the investigation of the subject of the transformation of heat into mechanical energy. His manner of operating was strikingly original and one of great merit, and has assisted wonderfully in the development of this part of science since his time. He established two new and distinct propositions in connection with his method:
1. That we have no right to reason on what has taken place in any series of operations till the working substance has been brought back to its initial state, nor to assign any relation between heat and work by such reasoning.
2. That a reversible engine is the most perfect engine possible. And, consequently, if we possessed a reversible engine, and a condenser absolutely deprived of heat, we could convert the whole of the heat from the boiler into mechanical energy. But since it is impossible to obtain an absolutely cold condenser, there is always a large fraction lost in attempting to convert heat into mechanical energy. Sir W. Thomson, working from the principle laid down by Carnot, found that the heat taken in by a perfect engine is to that given out as the absolute temperature of the boiler is to that of the condenser. He also, carrying the process further, devised a correct proof of Carnot's second proposition, based upon the axiom that "it is impossible, by means of inani-
