MOTIVE POWER OF HEAT. 199 



boiler be 3^ atmospheres (temperature 140) the 

 amount of work for each unit of heat will be 

 found, by dividing this by 618, to be 130.7 foot- 

 pounds, which is -VA 5 or 29.7 per cent of the theo- 

 retical duty.* 



(5) The 'actual average of work performed by 

 good Cornish engines and boilers is 55,000,000 

 foot-pounds for each bushel of coal, or less than 

 half the experimental performance of the Fowey 

 Consols engine, more than half the actual duty 

 performed by the United Mines engine in 1840; 

 in fact, about 25 per cent of the theoretical duty. 



(6) The average performances of a number of 

 Lancashire engines and boilers have been recently 

 found to be such as to require 12 Ibs. of Lanca- 

 shire coal per horse-power per hour (i.e., for per- 

 forming 60 X 33,000 foot-pounds), and of a num- 

 ber of Glasgow engines such as to require 15 Ibs. 

 (of a somewhat inferior coal) for the same effect. 

 There are, however, more than twenty large en- 

 gines in Glasgow at presentf which work with a 



* If, in this case again , the pressure required in the boiler 

 to make the engine work according to the contract were 

 only 15 Ibs. on the square inch, we should have a different 

 estimate of the economy, for which see Table B, at the 

 end of this paper. 



f These engines are provided with separate expansion 



