THE.RMO-D YNAMICS. 43 



temperatures ; suppose this vessel filled with ice, and here 

 I have the room at a certain different temperature. Let me 

 start with this cylinder, and suppose that the temperature 

 was 10, and I increase the heat to the condition indicated 

 by the point C on the diagram. Let me now compress this 

 suddenly so that no heat escapes. I bring it to the point A ; 

 then while it is still compressed very suddenly I have it at 

 a high temperature. I ought to have pointed out that 

 the temperature in the vessel was 10, and that by s-uddenly 

 compressing it I raised it up to a temperature of 20 at a 

 great pressure ; then you may diminish the pressure 

 gradually, keeping it at a constant temperature. Then it 

 will expand until it occupies some position on that line 

 indicated by B. Then let me suddenly expand it a good 

 bit more without allowing any heat to escape ; it will 

 grow cool because it has exerted a mechanical effect in 

 driving out the piston, and since no heat escapes its 

 condition will pass along this line and it will come to the 

 point D. Let this go on until it reaches the temperature 

 of this vessel, 10; then finally let me put it in this vessel, 

 and keeping it at a constant temperature gradually increase 

 the pressure until I reach the original volume which it 

 occupied, which was indicated by the point C. Thus I 

 have completed a cycle of operations which you will very 

 easily see is perfectly reversible, because I have only taken 

 in or given out heat during two of the operations, when 

 passing from A to B, and from D to C. I took in heat when 

 passing from A to B, and gave it out when passing from 

 D to C. Along the other parts of the curve there was no 

 transference of heat, because I performed the operation so 

 suddenly. Now since the transference of heat always took 

 place at a definite temperature, without the temperature 

 changing, it is perfectly easy to reverse that operation and 

 to pass from B to A by compressing the substance gradually 

 without any heat being allowed to escape. This is exactly 

 the reverse operation which was performed before. You 

 will easily see therefore that all these four operations are 

 perfectly reversible, and therefore such an engine, if it 

 could be made, would be a perfectly reversible engine. 



Now the remarkable point connected with this reversible 

 engine is, that a reversible engine is the most perfect kind 

 of engine that can be imagined. If it were possible to 



