Thermodynmnics of Fluids. 



319 



Fig. 3. 

 A 



the pressures. Hence, in thermodynamic problems, it is generally 

 necessary to distinguish between the quantities of heat received or 

 given out by the body at difterent tem])eratures, wliile as far as work 

 is concerned, it is generally sufficient to ascertain the total amount 

 performed. If, then, several heat-areas and one work-area enter into 

 the problem, it is evidently more important tliat the former should be 

 simple in form, than that the latter should be so. Moreover, in the 

 very common case of a circuit, the work-area is bounded entirely by 

 the path, and the form of the isometrics and tlie line of no pressure 

 are of no especial consequence. 



It is worthy of notice that the siinplest form of a perfect tliermody- 

 namic engine, so often descril)ed in treatises on tliermodynamics, is 

 represented in the entropy-tempera- 

 ture diagram by a figure of extreme 

 simplicity, viz : a rectangle of whicli 

 the sides are parallel to the co-ordi- 

 nate axes. Thus in figure 3, the 

 circuit ABCD may represent the se- 

 ries of states through which the fluid 

 is made to pass in sucli an engine, 

 the included area representing the 

 work done, while the area ABFE '^ E F ?/ 



represents the heat received from the heater at the highest tempera" 

 ture AE, and the area CDEF represents the lieat transmitted to the 

 cooler at the lowest temperature DE. 



There is another form of the perfect thermodynamic engine, viz: 

 one with a perfect regenerator as defined by llankinc (Phil. Trans, 

 vol. 144, p. 140), the representation 

 of which becomes peculiarly simple 

 in the entropy-temperature diagram. 

 The circuit consists of two equal 

 straight lines AB and CD (fig. 4) 

 parallel to the axis of abscissas, and 

 two precisely similar curves of any 

 form BC and AD. The included 

 area ABCD represents the work 

 done, and the areas ABba and CDdc 



represent respectively the heat re- ^ ' 



ceived from the heater and that transmitted to the cooler, 

 imparted by the fluid to the regenerator in passing from B to C, and 

 afterward restored to the fluid in its ]>assage from D to A, is repre- 

 sented by the areas BCcb and DAad. 



Fia-. 4. 



b 1] 



The lieat 



