832 THE POPULAR SCIENCE MONTHLY. 



about two pounds of coal per hour per horse-power. Hence it is said 

 that the steam-engine is a very inefficient machine, which the genius of 

 the future inventor may improve till a pound of coal is made to yield 

 six, eight, or ten times the power it now gives. 



But, while the theory of thermodynamics asserts that the com- 

 bustion of a pound of carbon yields an amount of heat equivalent to 

 11,116,800 foot-pounds of energy, it also asserts just as clearly and 

 just as firmly that, under the conditions which exist upon this planet, 

 heat can not be transformed into mechanical effect without wasting a 

 considerable portion of the energy due to it. In other words, if a given 

 quantity of heat be taken from a source, as a steam-boiler, and made 

 to do work by means of any form of engine, a considerable portion of 

 that heat must, from the very nature of things under existing condi- 

 tions, be allowed to pass through the engine and raise the temperature 

 of other bodies to no useful purpose. It is to the discussion upon 

 which this conclusion is based that I propose to devote this article. 



In a very valuable essay published in 1824, Cai'not furnished us 

 with a conception which is an exceedingly useful one in this investiga- 

 tion. It is that of an engine completely reversible in all its physical 

 and mechanical agencies. A water-motor which could be driven back- 

 ward by means of some source of power, and would then raise to a 

 given level as much water as it would use from that level to develop 

 the power required to run it backward, would be a reversible engine. 

 It is plainly to be seen that such a motor would be a perfect motor, 

 and also a perfect pump. A reversible heat-engine would be one 

 which, running forward and performing a certain amount of work by 

 means of a given amount of heat derived from a source, would, if run 

 backward by the performance upon it of the same amount of work, 

 restore to the source the same amount of heat. 



A reversible engine in this sense is, of course, impossible in prac- 

 tice, but the theoretical deductions from the conception are in no way 

 invalidated by this fact. Such a heat-engine would be a perfect engine 

 in the sense that it would produce as much mechanical effect as could 

 be produced by any heat-engine under the same conditions from the 

 same quantity of heat. The proof of this proposition rests upon two 

 assumptions that are supported by all past experience, and may, there- 

 fore, be regarded as physical axioms : 1. That a perpetual motion is 

 impossible ; 2. That it is impossible by means of inanimate material 

 agency to derive mechanical effect from any portion of matter by cool- 

 ing it below the temperature of surrounding objects. 



A heat-engine when at work must carry heat from a body of high 

 temperature (the source) to one of low temperature (the refrigerator), 

 and experiment has proved that the amount of heat given to the re- 

 frigerator is always less than that taken from the source. Now, let 

 there be two heat-engines, A and B, of which B is reversible, working 

 between the same source and refrigerator. Let each take the same 







