148 THOMSON ON CARNOT'S 



A^A^N^N^. Again, during the third operation a 

 certain amount of work is spent on the piston^ 

 which will be represented by the area A^A^N^N^ ; 

 and lastly, during the fourth operation, work is 

 spent in pushing the piston to an amount repre- 

 sented by the area A 3 A ON 3 . 



19. Hence the mechanical effect (represented 

 by the area OA A A^N^) which was obtained dur- 

 ing the first and second operations, exceeds the 

 work (represented by N^A^A^AO) spent during 

 the third and fourth, by an amount represented 

 by the area of the quadrilateral figure AA 1 A. 2 A 3 ; 

 and, consequently, it only remains for us to 

 evaluate this area, that we may determine the 

 total mechanical effect gained in a complete 

 cycle of operations. Now, from experimental data, 

 at present nearly complete, as will be explained 

 below, we may determine the length of the line 

 AA l for the given temperature S, and a given ab- 

 sorption H, of heat, during the first operation; 

 and the length of A^A Z for the given lower tem- 

 perature T, and the evolution of the same quantity 

 of heat during the fourth operation: and the 

 curves A^PA^, A 3 P'A may be drawn as graphical 

 representations of actual observations. The figure 

 being thus constructed, its area may be measured, 

 and we are, therefore, in possession of a graphical 



