572 PROFESSOR WILLIAM THOMSON S ACCOUNT OF 



duty reported in the case of the Fowey Consols engine. Hence, the best useful 

 worli on record, is at the rate of 198'3 foot-pounds for each unit of heat transmit- 

 ted, and is 'j^\ or 45 per cent, of the theoretical duty, on the supposition that the 

 boiler is at 140', and the condenser at 30". 



(3.) French engineers contract (in Lille, in 1847, for example) to make en- 

 gines for mill power which will produce 30,000 metre-lbs., or 98,427 foot-lbs. of 

 work for each pound of steam used. If we divide this by 618, we find 1.59 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 3| lbs. of the best coal per horse-power per hour. Hence, in 

 such engines, each pound of coal ought to produce 505,700 foot-pounds of work, 

 and if 7 lbs. of water be evaporated by each pound of coal, there would result 

 80,814 foot-pounds of Avork for each pound of water evaporated. If the pressure 

 in the boiler be 3i atmospheres (temperature 140°) the amount of Avork for each 

 unit of heat will be found, by dividing this by 618, to be 130-7 foot-pounds, which 

 is 1^ or 29-7 per cent, of the theoretical duty.f 



(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 ex- 

 perimental 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 lbs. of Lancashire coal per 

 horse-power per hour (i. e., for pei'forming 00 x 33,000 foot-pounds) and of a num- 

 ber of Glasgow engines, such as to require 15 lbs. (of a somewhat inferior coal) 

 for the same effect. There are, however, more than twenty large engines in Glas- 

 gow at present, j which work with a consumption of only 6^ lbs. of dross, equiva- 

 lent to 5 lbs. of the best Scotch, or 4 lbs. of the best Welsh coal, per horse-power 



* It being assumed that the temperatui-es of the boiler and condenser ai-e the same as those of 

 the Cornish engines. If, however, the pressure he lower, two atmospheres, for instance, the num- 

 bers would stand thus : The temperatvire in the boiler would be only 121. Consequently, 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 temperatures, or the fall utilised, is 

 only from 131 to 30, instead of from 140 to 30°, and, consequently (Table II.), the theoretical duty 

 for each unit of heat is only 37 1 foot-pounds. Hence, if the engine, to work according to the speci- 

 fication, requires a pressure of only 15 lbs. on the square inch (i. «., a total steam pressure of two at- 

 mospheres), its performance is '|*'', or 43'2 per cent, of its theoretical duty. 



■f If, in this case again, the pressure required in the boiler to make the engine work according 

 to the contract were only 15 lbs. on the square inch, we should have a different estimate of the eco- 

 nomy, for which, see Table B, at the end of this paper. 



I These engines are provided with separate expansive cylinders, which have been recently added 

 to them by Mr M'Naught of Glasgow. 



