the Economy of Fuel. 113 



yet made ; and, as they were all conducted with care, 

 their results have intrinsic value independent of the 

 particular uses to which I have applied them. 



As, in the countries where this Essay is likely to be 

 most read, pit-coals are more frequently used as fuel 

 than wood, it will not only be satisfactory, but in many 

 cases may be really useful, to my reader to know the 

 relative quantities of heat producible from coals and 

 from wood, in order to be able to compare the results 

 of experiments in which coals are used as fuel, with 

 those of which I have here given an account ; or to 

 determine the quantity of coals necessary in any pro- 

 cess which it is known may be performed with a given 

 quantity of wood. 



It was my intention to have made a set of experi- 

 ments on purpose to determine the relative quantities 

 of heat producible from all the various kinds of combus- 

 tible bodies which are used as fuel ; and I made pre- 

 parations for beginning them, but I have not yet been 

 able to find leisure to attend to the subject. 



The most satisfactory account I have been able to 

 procure respecting the matter in question is one for 

 which I am indebted to my friend Mr. Kirwan. By this 

 account, which he tells me is founded on experiments 

 made by M. Lavoisier,' it appears that equal quantities 

 of water, under equal surfaces, may be evaporated, and 

 consequently equal heats produced 



In weight, In measure, 



By 403 Ibs. of cokes, By 17 of cokes, 

 600 of pit-coal, 10 of pit-coal, 



600 ,, of charcoal, 40 of charcoal, 



1089 of oak ; 33 of oak. 



I wish I were at liberty to transcribe the ingenious 

 and interesting observations which accompanied this 



