and on the Mechanical Value of Heat. 441 



In other words, one degree of heat per lb. of water may be 

 generated by the expenditure of a mechanical power capable 

 of raising 742 lbs. to the height of one foot. 



By a similar calculation, I find the result of the last two ex- 

 periments of the table to be 860 lbs. 



The foregoing are all the experiments I have hitherto made 

 on the mechanical value of heat. I admit that there is a con- 

 siderable difference between some of the results, but not, I 

 think, greater than may be referred with propriety to mere 

 errors of experiment. I intend to repeat the experiments with 

 a more powerful and more delicate apparatus. At present we 

 shall adopt the mean result of the thirteen experiments given 

 in this paper, and state generally that, — 



The quantity of heat capable of increasing the temperature 

 of a pound of water by one degree of Fahrenheit* s scale is equal 

 to, and may be converted into, a mechanical force capable of 

 raising 838 lbs. to the perpendicular height of one foot. 



Among the practical conclusions which may be drawn from 

 the convertibility of heat and mechanical power into one an- 

 other, according to the above absolute numerical relations, I 

 will content myself with selecting two of the more important. 

 The former of these is in reference to the duty of steam-en- 

 gines ; the latter, to the practicability of employing electro- 

 magnetism as an ceconomical motive force. 



1 . In his excellent treatise on the Steam-engine, Mr. Rus- 

 sell has given a statistical table*, containing, among other im- 

 portant matter, the number of pounds of fuel evaporating one 

 cubic foot of water, from the initial temperature of the water, 

 and likewise from the temperature of 212°. From these facts 

 it appears that in the Cornish boilers at Huel Towan, and the 

 United Mines, the combustion of a lb. of Welsh coal gives 

 183° to a cubic foot of water, or otherwise 11,437° to a lb. of 

 water. But we have shown that one degree is equal to 838 

 lbs. raised to the height of one foot. Therefore the heat 

 evolved by the combustion of a lb. of coal is equivalent to the 

 mechanical force capable of raising 9,584,206 lbs. to the 

 height of one foot, or to about ten times the duty of the best 

 Cornish engines. 



2. From my own experiments, I find that a lb. of zinc con- 

 sumed in Daniell's battery produces a current evolving about 

 1320°; in Grove's battery, about 2200° per lb. of water. 

 Therefore the mechanical forces of the chemical affinities which 

 produce the voltaic currents in these arrangements, are, per lb. 

 of zinc, equal respectively to 1,106,160 lbs. and 1,843,600 lbs. 

 raised to the height of one foot. But since it will be practically 

 impossible to convert more than about one half of the heat of 



* Enc. Brit., 7th Edition, vol, xx. part 2. p. 685. 



