838 Transactions of the American Institute. 



nary shape and proportions, it is sufficient to allow fifteen per cent 

 /or medium size, and as low as ten per cent or eyen eight per cent, in 

 exceptional cases, in large engines of solid construction and good work- 

 manship. 



Having described the instruments used in determining the power 

 of the steam engine, we propose to postpone future remarks upon the 

 proper methods of their application and use, until the closing general 

 discussions ; and we will now proceed with the next branch of inquiry ; 

 namely : 



The Economy ok Cost of the Power. 



Money is the standard unit of value. Hence, everything which 

 costs money, that is required in order to obtain the steam ])ower in 

 any case, is a proper charge to tlie cost of the power. Therefore, 

 strictly speaking, the cost of the fuel, of the oil, and of needed repairs, 

 together with the wages of the attendants, and also, perhaps a sink- 

 ing fund for prospective renewals, should all form part of the aggre- 

 gate cost. Nor should either of these items be neglected. It would 

 be poor economy for a person to purchase an engine designed to save 

 fuel, which, for any reason was liable to frequent derangement ; for 

 it is not alone the cost of the repairs which are to be considered, 

 but the losses which occur from stopping work in the mill or factory. 

 We cannot, however, in our present inquiry, discuss matters of design 

 (though they should always be considered by a purchaser), but must 

 confine ourselves to the methods and means employed to ascertain 

 the economy of fuel. 



The combustion of the fuel evolves heat, which uses water as a 

 vehicle, and is carried with it to the engine, and there produces the 

 power. The true measure of the cost, then, is the quantity of heat 

 required to perform a certain quantity of work. Heat being 

 imponderable, can be measured only by its effects on other bodies. 

 The standard unit of heat, or " heat miit," is the heat required to 

 raise the temperature of one pound of distilled water at thirty-nine 

 degrees one degree Fahrenheit. The meclianical equivalent of a unit 

 of heat is 772 foot-pounds of work ; but the best steam engines 

 obtain only about one-tenth of that quantity. Such a result has 

 often been regretted by scientific minds, and many have spoken of it 

 as mysterious. We consider the steam engine of to-day very defec- 

 tive. Some of the defects are inherent; they can be pointed out, but 

 cannot be remedied without clianging the gcnei^al principles of 



