connected with the Theory of the Steam-Engine. 387 



work, a part of it is condensed, and that this condensation 

 affords heat for the expansion of the remainder of the steam. 

 This fact, which is analogous to that of the production of a 

 cloud when air saturated with vapour is rarefied in the re- 

 ceiver of an air-pump, explains the approach of the cecono- 

 mical duty of the steam-engine to that of the air engine, on 

 which I propose to make a few observations shortly. 

 I have the honour to remain, Gentlemen, 



Yours very respectfully, 



James P. Joule. 



My DEAR Sir, Paris, October 15, 1850. 



In Mr. Rankine's paper on the Mechanical Action of Heat*, 

 the following very remarkable resultisannounced: — "If vapour 

 at saturation is allowed to expand, and at the same time is 

 maintained at the temperature of saturation, the heat which 

 disappears in producing the expansion is greater than that set 

 free by the fall of temperature, and the deficiency of heat must 

 be supplied from without, otherwise a portion of the vapour 

 will be liquefied in order to supply the heat necessary for the ex- 

 pansion of the rest" This conclusion can, I think, be recon- 

 ciled with known facts only by means of your discovery, that 

 heat is evolved by the friction of fluids in motion. For it 

 is well known that the hand may be held with impunity in 

 a current of steam issuing from the safety-valve of a high- 

 pressure boiler; and again, it is known that "Watt's law" 

 does not rigorously express the actual decrease in the latent 

 heat of saturated steam with an elevation of temperature; but, 

 on the contrary, Regnault shows that the "total heat" of satu- 

 rated steam increases slowly with the temperature, at an ap- 

 proximately uniform rate. These two facts are consistent and 

 connected with one another; for, according to the latter, steam 

 issuing from a high-pressure boiler ought, in the immediate 

 neighbourhood and on the outside of the orifice, where, of 

 course, its pressure scarcely exceeds that of the atmosphere, to 

 be at a temperature sensibly above 212°, and consequently su- 

 persaturated, and quite dry ; and it is well known that the hand 

 experiences no pain from being exposed to a hot current of a 

 dry gas, even if the temperature considerably exceeds 212°. 

 But, according to Mr. Rankine's proposition, steam allowed 

 to expand from saturation will, if no heat be supplied to it, re- 

 main saturated, except a small portion which becomes liquefied. 

 Either then Mr. Rankine's conclusion is opposed to the facts, 



* Transactions of the Royal Societv of Edinburgh, vol. xx. part 1. (Read 

 Feb. 4, 1850.) 



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