MOTIVE POWER OF HEAT. 



147 



last point of this curve must, according to CarnoVs 

 fundamental principle, coincide with A, since the 

 piston is, at the end of the cycle of operations, 



again in its primitive position, and the pressure of 

 the vapor is the same as it was at the beginning. 



18. Let us now suppose that the lengths, ON^ , 

 JVjJV,, N^NI, and N t O, represent numerically the 

 volumes of the spaces moved through by the piston 

 during the successive operations. It follows that 

 the mechanical effect obtained during the first 

 operation will be numerically represented by the 

 area AA^N^O; that is, the number of superficial 

 units in this area will be equal to the number of 

 " foot-pounds" of work performed by the ascend- 

 ing piston during the first operation. The work 

 performed by the piston during the second opera- 

 tion will be similarly represented by the area 



