198 THOMSON ON CAENOT'S 



we divide this by 618, we find 159 foot-pounds for 

 the work produced by each unit of heat. This is 

 36.1 per cent of 440, the theoretical duty.* 



(4) English engineers have contracted to make 

 engines and boilers which will require only 3J Ibs. 

 of the best coal per horse-power per hour. Hence 

 in such engines each pound of coal ought to pro- 

 duce 565,700 foot-pounds of work, and if 7 Ibs. of 

 water be evaporated by each pound of coal, there 

 would result 83,814 foot-pounds of work for each 

 pound of water evaporated. If the pressure in the 



* It being assumed that the temperatures of the boiler 

 and condenser are the same as those of the Cornish en- 

 gines. If, however, the pressure be lower, two atmos- 

 pheres, for instance, the numbers would stand thus: The 

 temperature in the boiler would be only 121. Conse- 

 quently, for each pound of steam evaporated, only 614 

 units of heat would be required ; and therefore the work 

 performed for each unit of heat transmitted would be 

 160.3 foot-pounds, which is more than according to the 

 estimate in the text. On the other hand, the range of tem- 

 peratures, or the fall utilized, is only from 131 to 30, in- 

 stead of from 140 to 30, and, consequently (Table II.), the 

 theoretical duty for each unit of heat is only 371 foot- 

 pounds. Hence, if the engine, to work according to the 

 specification, requires a pressure of only 15 Ibs. on the 

 square inch (i.e., a total steam-pressure of two atmos- 

 pheres), its performance is - l $%', or 43.2 per cent of its 

 theoretical duty. 



