MOTIVE POWER OF SEAT 163 



kind we have considered, and thus to continually 

 restore the heat from B to A, which has descended 

 from A to B for working itself; so that we should 

 have a complex engine, giving a residual amount 

 of mechanical effect without any thermal agency, 

 or alteration of materials, which is an impossibility 

 in nature. The same reasoning is applicable to 

 the air-engine ; and we conclude, generally, that 

 any two engines, constructed on the principles laid 

 down above, whether steam-engines with different 

 liquids, an air-engine and a steam-engine, or two 

 air-engines with different gases, must derive the 

 same amount of mechanical effect from the same 

 thermal agency. 



30. Hence, by comparing the amounts of me- 

 chanical effect obtained by the steam-engine and 

 the air-engine from the letting down of the H 

 units of heat from A at the temperature (t -j- *) to 

 B at t, according to the expressions (2) and (3), 

 we have 



M =(l-a)%L.ffT = ^j-.HT.. (5) 



' kdt vdq/dv 



If we denote the coefficient of Hr in these equal 

 expressions by //, which maybe called '"Carnot's 

 coefficient," we have 



