and the Laws regarding the Nature of Heat. 



13 



owt 



abscissa oe, and the pressure of ria^di ^d VfiFig^i'SJK^'P^ erilT 

 the steam by the ordinate ea. Let ^IdBiiiqtjs laimm »vod*? ad* 1<> 

 the vessel be now supposed to r 

 expand, whilebothfluidand steam 

 are kept in contact with a body 

 A of the constant temperature < Ik rtJ^v ov/ 

 t. As the space increases, more t ijijt rijuli^ 

 fluid is evaporated, the necessary 'fjf'p') '^f t V' -^l 

 amount of latent heat being sup- 

 plied by the body A ; so that the 

 temperature, and consequently 

 the pressure of the steam, may remain unchanged. "When the 

 entire volume is increased in this manner from oe to o/, an ex- 

 terior work is produced which is represented by the rectangle ea 

 bf. Let the body A be now taken away, and let the vessel con- 

 tinue to expand without heat being either given or received. 

 Partly by the expansion of the steam already present, and partly 

 by the formation of new steam, the temperature will be lowered 

 and the pressure become less. Let the expansion be suiFered to 

 continue until the temperature passes from t to r, and let og 

 represent the volume at this temperature. If the decrease of 

 pressure during this expansion be represented by the curve he, 

 the exterior work produced by it will be represented by fbcg. 



Let the vessel be now pressed together so as to bring the fluid 

 and vapour to their original volume oe, and during a portion of 

 the process let the vessel be in contact with a body, B, of the 

 temperature t, to which any excess of heat shall be immediately 

 imparted, and the temperature of the fluid and vapour kept con- 

 stant at T. During the other portion of the process, let the body 

 B be withdrawn so that the temperature may rise ; let the first 

 compression continue till the volume has been reduced to oh, it 

 being so arranged that the remaining space he shall be just suf- 

 ficient to raise the temperature from r to t. During the first 

 decrease of volume the pressure remains constant at gc, and the 

 quantity of exterior work expended is equal to the rectangle ^c dh. 

 During the last decrease of volume the pressure increases, and 

 may be represented by the curve da, which must terminate exactly 

 in the point a, as the original temperature t must again corre- 

 spond to the original pressure ea. The exterior work expended 

 in this case is :=-hdae. 



At the end of the operation both fluid and vapour are in the 

 same state as at the commencement, so that the excess of the 

 exterior work produced over the amount expended expresses the 

 total amount of work accomplished. This excess is represented 

 by the quadrilateral ahcd, the content of which must therefore 

 be compared with the heat expended at the same time. 



