44 LECTURES TO SCIENCE TEACHERS. 



have an engine more perfect than this reversible engine, 

 let me employ that engine to drive this reversible one, of 

 which I have been speaking, in the reverse direction. 

 Then, since it is more powerful than this reversible engine, 

 it will transfer more heat from this cool part from every 

 part of it beyond C D to the body, which is at a tempe- 

 rature of 20 ; and therefore the most perfect engine will 

 cause this reversible engine to transfer more heat from 

 C D to A B than that more perfect engine is utilizing ; in 

 other words, we should have perpetual motion by the 

 employment of such an engine. The more perfect engine 

 would be using this reversible engine in order to store up 

 a large supply of heat, to carry heat from a body at a 

 low temperature to a body at a high temperature. 



Now we may state the second law of the 4 rmo-dynamics, 

 either in the form that it is impossible by the aid of any 

 dead matter to transfer heat from a source at a low 

 temperature to a body at a high temperature ; or we may 

 say that it is impossible to produce perpetual motion. In 

 either of these cases the reversible engine must be the 

 most perfect one that we can produce. I am afraid that I 

 shall nob have time to go as fully into the diagram as I 

 intended, but I will indicate some points about it which 

 lead to most important conclusions. In the reversible 

 engine the path A B C D is traced out, and we can also 

 prove very simply that the area A B C D is proportional 

 to the amount of work which is done by the engine. It is 

 impossible for me to explain to you the reasoning by which 

 Sir William Thomson arrived at this conclusion, but I will 

 simply allude to the fact that Sir William Thomson has 

 found that if we measure the temperature of a body by 

 the area included between these two adiabatic lines from 

 the points where they meet one another up to the isothermal 

 line for that particular temperature, then we have an 

 absolute measure of temperature which is independent of 

 the substance which is employed to make our thermometer 

 of ; and also that this measure of temperature agrees very 

 closely with the measure derived from an air thermometer. 



Consequently it follows that the heat utilized in such an 

 engine as I have been describing is to the heat which is 

 absorbed from the boiler in the ratio of the difference of 

 temperature between the refrigerator and the boiler to the 



