840 Transactions of tue American Institute. 



entitled to the credit of the performance. This standard will not then 

 answer the purpose of a scientific investigation. In such case, we 

 must ascertain, in addition to the coal, the amount of water evapo- 

 rated ; we can then estimate the value of the coal, and the separate 

 etticiencj of both the engine and boiler. The value of the coal, and 

 the efficiency of the boiler, are shown by the number of pounds of 

 water evaporated per pound of coal, and the economy' of the engine 

 as compared with that of others by calculating the number of pounds 

 of steam used per horse-power per hour. The weight of the steam 

 used is, of course, the same as that of the water evaporated. 



In all ordinary practical trials, the economy must be determined 

 simply by the quantity of fuel consumed to produce the power. 

 Hence, we will first try and find a solution of the difiiculties which 

 attend this kind of measurement. 



The Fuel. 



The different kinds of fuel vary so much in value that it is impos- 

 sible to accurately compare them. Coal being most generally used, 

 is . the natural standard ; but there are so many varieties of this 

 necessary article, varying greatly in quality, that it seems a hopeless 

 task to try and compare the performance of steam engines in different 

 parts of the world, or even of our own countr}-, by the consumption 

 of diflfering coal, which may vary twenty per cent in heat-producing 

 power. The best way is, evidently, in comparative trials, to use 

 selected coal from the same mine. Yet, how rarely can this be done ; 

 and even if this precaution be taken in certain cases, how can a 

 'comparison be made with the results obtained by others widely 

 separated, and possessing, possibly, diftering views ? We must say 

 that the problem cannot be solved with scientific accuracy ; still we 

 are able to suggest some corrections which will reduce all varieties of 

 good coal to substantially the same standard, and thus unable us to 

 use this measure in simple practical trials. 



"We cannot examine in this paper, with any minuteness, the chemi- 

 cal constituents of the different varieties of coal. For our purpose, 

 we will simply divide them into two portions : namely, the non-com- 

 bustible and combustible. 



The non-combustible portion consists, for the most part, of earthy 

 matters, though oxygen and nitrogen gases are often present ; and 

 most coals absorb considerable water. The combustil)le portion con- 

 sists of carbon and hydrogen, the first largely predominating. In 



