2 W. J. M. Rankine on the Means of 



The general rules according to which these phenomena take 

 place, have for some time been determined empirically, with 

 more or less precision, for certain particular substances — for 

 example, for steam ; but all systematic knowledge respecting 

 them, as they affect all substances whatsoever, is deducible 

 from two laws — that of the mutual convertibility of heat and 

 mechanical power, and that of the efficiency of thermo- 

 dynamic engines; the term ther mo -dynamic engine being 

 used to denote any body, or assemblage of bodies, which pro- 

 duces mechanical power from heat. 



2. The Law of the Mutual Convertibility of Heat 

 and Mechanical Power is this : That when mechanical 

 power is produced by the expenditure of heat, a quantity of 

 heat disappears bearing a fixed proportion to the power 

 produced ; and conversely, that when heat is produced by 

 the expenditure of mechanical power, the quantity of heat 

 produced bears a fixed proportion to the power expended. 



This law was believed, and reasoned from, by some inquirers 

 before it was proved by experiment ; but being inconsistent 

 with the formerly prevalent hypothesis of the existence of a 

 peculiar substance as the cause of the phenomena of heat, it 

 was recognised by few, until Mr Joule, by experiments on the 

 production of heat by the friction of the particles of various 

 substances, solid, liquid, and gaseous, not only demonstrated 

 the mutual convertibility of heat and mechanical power, but 

 ascertained the fixed proportion which they bear to each 

 other in cases of mutual conversion, which is this : The unit 

 of heat generally employed in Britain — that is to say, so 

 much heat as is sufficient to raise the temperature of one 

 pound of water, at ordinary temperatures, by one degree of 

 Fahrenheit's thermometer — requires for its production, and 

 produces by its disappearance, in other words, is equivalent to, 

 772 foot-pounds of mechanical power ; that is to say, so much 

 mechanical power as is sufficient to lift a weight of one pound 

 to a height of 772 feet.* 



* The value of Joule's equivalent for a degree of the centigrade scale is 

 772x2 = 1389-6 foot-pounds. For French measures, viz., centigrade degrees, 

 kilogrammes, and metres, it is 423*54 kilogrammetres. See Philosophical 

 Transactions, 1850. 



