4 Mr. J. P. Joule on the ceconomical production of 



forces atmospheric air into the receiver, in the receiver its elas- 

 ticity is increased by the application of heat, and then the air 

 enters the expansion cylinder, of which the volume is to that of the 

 pump as the absolute temperature of the air in the receiver is to 

 that of the air entering it. The cylinder is furnished with expan- 

 sion gear to shut oflP the air, when the same quantity has been 

 expelled from the receiver as was forced into it by one stroke of 

 the pump. By this disposition the air is expelled from the 

 expansion cylinder at the atmospheric pressure, and at the 

 absolute temperature corresponding with b in Professor Thom- 

 son's formula. 



As an example of the above kind of air-engine, I will take 

 one working in atmospheric air of 15 lbs. pressure on the 

 square inch and 50° Fahr. I will suppose that the expansive 

 action in the cylinder is to exist through three-fourths of its 

 length. Then as the action of the compressing pump is the 

 reverse of that of the cylinder, the piston of the former must 

 traverse three-fourths of its length before the air is sufficiently 

 compressed to enter the receiver by its own pressure. The 

 temperature of the air entering the receiver, determined by 



Poisson's equation -=1:^,] , will be 439°*59 Fahr., and its 



pressure will be 105*92 lbs. on the square inch. Supposing 

 now that the volume of the cylinder is to that of the pump as 4 

 to 3, the density of the air in the receiver to that forced into it 

 by the pump must be as 3 to 4, in order to keep the quan- 

 tity of air in the receiver constant. The temperature of the 

 air in the receiver will also require to be kept at 739°* 12 Fahr. 

 in order to maintain the pressure of 105 '92 lbs. on the square 

 inch. The air entering the cylinder at the above pressure and 

 temperature will escape from it at the end of the stroke at the 

 atmospheric pressure, and at the temperature 219f°. 



It will be remarked that there are two ranges of temperatures 

 in the engine 1 have described, viz. that of the pump, and that 

 of the cylinder. Owing, however, to the exact proportion which 

 subsists between the two, the same result is arrived at by the 

 application of Professor Thomson's formula to either of them. 

 Taking, therefore, the range of the cylinder, and converting the 

 temperatures of the air entering and discharged from the cylinder 

 into the absolute temperatures from the real zero by adding to 

 them 459°, we obtain for the work evolved by the consumption 

 of a grain of coal, 



«r 1261-45 (1198-12— 678-66) . .^ oo ^ , i 



yy = iMXQii^ = 546-92 foot-pounds. 



