466 POPULAR LECTURES AND ADDRESSES. 



backwards with every action in its cycle exactly 

 reversed. When working forwards it performs 

 mechanical work in virtue of heat taken from a 

 hot body, A, of which a certain portion is essen- 

 tially given to a body, B, at a lower temperature. 

 To reverse its action mechanical work must be 

 done upon it, and the equivalent output is a 

 certain quantity of heat taken from the cold body, 

 B, and a greater quantity given to the hot body, 

 A. The excess of the quantity of heat taken 

 from A above that given to B when the engine 

 works forwards, and the excess of the heat given 

 to A above that taken from B when the engine 

 is worked backwards, is equal to the quantity 

 of heat which has the same dynamical energy 

 as the work done by the engine, in the case of 

 working forwards and the work done upon tlic 

 engine by an external, agent, when the engine is 

 worked backwards. 



It is impossible to fulfil the condition of perfect 

 reversibility by any engine composed of any real 

 .material to be found in nature. The friction of 

 the parts, and the impossibility of getting heat 



