246 THE POPULAR SCIENCE MONTHLY. 



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 transfor- 

 mation from kinetic to potential energy during the ascent, and a re- 

 transformation 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 over- 

 coming 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 fric- 

 tion. The energy so spent was long a puzzle to scientific minds, and 

 was believed to be absolutely destroyed, until the experiments of Rum- 

 ford 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 trans- 

 form the whole of the potential or kinetic energy of a body into heat ; 

 but it becomes quite a different undertaking when we propose to recon- 

 vert 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 sci- 

 ence 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 ab- 

 solutely 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 tem- 

 perature of the boiler is to that of the condenser. He also, carrying 

 the process further, devised a correct proof of Carnot's second propo- 

 sition, based upon the axiom that "it is impossible, by means qfinani- 



