MOTIVE POWER OF HEAT. 



115 



of utilizing a greater fall of caloric. The steam 

 produced under a higher pressure is found also 

 at a higher temperature, and as, further, the 

 temperature of condensation remains always about 

 the same, it is evident that the fall of caloric is 

 more considerable. But to obtain from high-pres- 

 sure engines really advantageous results, it is 

 necessary that the fall of caloric should be most 

 profitably utilized. It is not enough that the steam 

 be produced at a high temperature : it is also 

 necessary that by the expansion of its volume 

 its temperature should become sufficiently low. A 

 good steam-engine, therefore, should not only em- 

 ploy steam under heavy pressure, but under succes- 

 sive and very variable pressures, differ- 

 ing greatly from one another, and pro- 

 gressively decreasing.* 



In order to understand in some sort 

 a posteriori the advantages of high- 

 pressure engines, let us suppose steam 

 to be formed under atmospheric 

 pressure and introduced into the cylin- 

 drical vessel abed (Fig. 5), under the 

 piston cd, which at first touches the 

 bottom ab. The steam, after having FIG. 5. 

 moved the piston from ab to cd, will continue 



*Note G, Appendix B. 



