Proceedings of the Polytechnic Association. 843 



and American and bituminous coals are substantially equal in value, 

 the value of the combustible of the foreign coal will equal that of 

 American bituminoiTS and American anthracite, and we may assume 

 that the combustible of the coal, burned in any case, is a tolerably 

 accurate comparative measure of the economy of a steam engine. 

 All these restrictive qualiiications are necessary, for if selected coal 

 of the best quality, be used in a trial, the results will be above the 

 average in any case. We wish simply to indicate that the greatest 

 difference in the results given by diiferent coals is due to the differ- 

 ence in the quantity of non-combustible matter, so that, if this be 

 thrown out, the weight of the combustil)le remaining gives the near- 

 est approach possible, without absolute trial, to the comparative heat- 

 producing powers of different specimens. The best standard to show 

 the comparative economy of the steam engine, other than that of the 

 steam used, is therefore, '* The number of pounds of combustible used 

 per horse-power per hour." 



We cannot fairly, however, compare the combustible per horse- 

 power per hour, used in experiments here, with other experiments 

 where only the coal was noted. This necessitates us to correct the 

 amount of coal used by a common standard, founded on the com- 

 bustible. Good bituminous coals, here and in England, have about 

 ten per cent refuse ; hence, to make our experiments compare with 

 those abroad, as well as for convenience, we suggest that in every 

 case, the coal burned in determining the economy of a steam engine 

 be reduced to a common standard of ten per cent refuse. Let us see 

 the effect of this. The true comparative test for engines is the 

 amount of heat they receive. We have shown that the heat-pro- 

 ducing power of the coal is proportioned to the weight of the com- 

 bustible ; hence, if the weight of the coal be also proportioned to 

 that of the combustible, it also expresses the relative economy. The 

 coal is so proportioned when it leaves the same percentage of refuse ; 

 so by our plan of correcting the weight of the coal by its combusti- 

 ble, so as to give ten per cent refuse in each case, the weight of the 

 coal is a true comparative test of the relative economy of the engine. 

 For instance, one hundred pounds of coal, leaving twenty per cent 

 refuse, M'ill evaporate no more water than 88.9 pounds, leaving ten 

 per cent refuse, for both contain only eighty pounds of combustible. 

 If to the combustible we add one-ninth of its weiglit, the quantity 

 added is one-tenth or ten per cent of the sum, which represents the 

 weight of the coal, corrected to the uniform standard of ten per cent 



