1871.] Heat, and Force. 89 



utmost force which could be got out of steam, supposing 

 there were no waste ; but, unfortunately, on examining 

 the work actually done by existing steam engines, it was 

 found that they produced two or three times as much 

 work as he had calculated to be within the limits of possi- 

 bility. I find, also, that Sturgeon states that Professor Page 

 in America has produced from a galvanic battery nine times 

 as much work as Joule and Scoresby had proved to be the 

 utmost possible, according to their computation of the 

 mechanical equivalent of heat. And I much doubt whether 

 any philosophers have yet properly laid down the very first 

 principles of the question involved. No doubt there are 

 many cases in which, where the circumstances do not vary 

 much, it may be convenient to ascertain the usual amount 

 of energy derivable from a given amount of heat, and the 

 provisional assumption of the rule may be practically useful ; 

 but to proceed beyond this, and to lay down an universal 

 law, that heat has a definite and invariable mechanical value 

 is unphilosophical, and, to my own mind, inconsistent with 

 known facts. 



A given amount of heat applied to expand air will raise 

 ten times the weight that it will if applied to expand vapour 

 of turpentine, and one-and-a-half times as much as if it were 

 applied to expand steam. It may be answered, " Yes ; but it 

 also expands the vapour of turpentine or of water, as well as 

 raises the weight !" True, but this is not mechanical energy 

 as measured by foot-pounds raised ; and to assume that it 

 is equivalent to it is to beg the question at issue. 



7. I have shown that the very term " mechanical equiva- 

 lent of heat " is in itself fallacious. If there be any 

 mechanical equivalent ofthe kind it is not an equivalent of 

 heat, but of the disturbance or restoration of the equilibrium 

 of heat. But the question now arises, is there such a 

 maximum equivalent ? Or can we, by skill and contrivance, 

 increase indefinitely the amount of work to be got out of a 

 given disturbance or restoration of this equilibrium ? Take 

 two separate pounds of water, differing from each other and 

 from the temperature of the air by a given number of degrees 

 of temperature ; mix them together, and you get no work 

 out of them ; but connect them together by a bar of copper 

 of a temperature between the two, and you get one end of 

 the copper enlarged and the other diminished, and a series 

 of changes and motions going on, till all parts of the water 

 and copper at length arrive at an equal temperature. We 

 have now got some work out of them. Now put one at one 

 pole of a thermo-electric battery, and the other at the other 



VOL. VIII. (O.S.) — VOL. I. (N.S.) N 



