44 Mr Kenwood's account of the 



the average pressure of steam on the piston when the machine 

 is moving with its requisite velocity. This opinion is confirm- 

 ed by the knowledge, that the average performance of the ro- 

 tatory engines in Cornwall does not exceed seven millions, whilst 

 that of the pumping engines (in estimating the duty of which 

 no difficulty appears) is about thirty millions. That such a dif- 

 ference actually exists cannot be for one moment imagined. 

 The selection of one pound weight lifted one foot high, as a 

 dynamic unit, appears to have been very judicious. The term 

 efficiency seems so far useful, as its illustrious propounder had 

 intended ; but we think that consumption of fuel might also 

 be denominated expence of the efficient. Eighty-four appears 

 to us a very inconvenient unit. One pound would be much 

 more agreeable to our preconceived opinions of scientific order. 

 The only difference between the duty of a large and a small 

 engine, supposing them equally good, is only in the value of 

 the friction, which is inversely proportional to their dimen- 

 sions. Hence, supposing friction to vanish, 



Let r zz resistance. 



s zz space through which it is moved. 

 C = expence of the efficient. 



Then — - duty. 



Which under precisely similar circumstances, would be the 

 same for an engine of any dimensions ; the expence of the 

 efficient being always in proportion to the resistance overcome. 



