realizing the Advantages of the Air-Engine. 7 



upon the same portion of the substance, or upon a succession 

 of equal and similar portions, of a cycle of four processes, 

 which, taken together, constitute a single stroke of the engine.* 



Process A. The substance is raised to an elevated tempera- 

 ture. This process may or may not involve an alteration of 

 volume. 



Process B. The substance, being maintained at the elevated 

 temperature, increases in volume and propels a piston, or some- 

 thing equivalent to a piston. During this process heat dis- 

 appears, and a supply of heat from without is provided equal 

 in amount to the heat which disappears ; so that the tempera- 

 ture does not fall. 



Process C. The substance is cooled down to its original low 

 temperature. This process, like the process A, may or may 

 not be accompanied by a change of volume. 



Process D. The substance, being maintained at its depressed 

 temperature, is compressed, by the return of the piston, to its 

 original volume. During this process, heat is produced ; and 

 in order that it may not elevate the temperature of the sub- 

 stance, and give rise to an increased pressure, impeding the 

 return of the piston, it must be abstracted as quickly as pro- 

 duced, by some external means of refrigeration. 



The substance, being now brought back to its original volume 

 and temperature, is ready to undergo the cycle of processes 

 anew, and so on ad infinitum ; or otherwise, it is rejected, and 

 a fresh portion of the substance employed for the next stroke. 

 When the latter is the case, the operation of expelling the 

 substance from the engine into the atmosphere, by the return 

 of the piston, sometimes takes the place of the process D. 



Sometimes, either of the processes, B, C, or D, is the first in 

 the order of time. The cycle of processes, however, preserves 

 the same order of rotation. 



During the cycle of processes which has been described, the 

 working substance alternately increases and diminishes in 

 volume, in contact with a moving piston. During the increase 

 of volume, the pressure of the substance against the piston 



% This cycle of processes was first described by Carnot (Reflexions sur la 

 puissance motrice du feu) ; but his conclusions were vitiated by the assumptioD 

 of the substantiality of heat. 



