198 



Tuesday, January 2, 1849. 

 Dr CHRISTISON in the Chair. 

 The following Communications were read : — 

 1. An Account of Carnot's Theory of the Motive Power of 

 Heat,* with Numerical Results deduced from Regnault's 

 Experiments on Steam. t By Professor William Thomson, 

 of Glasgow. 



The questions to be resolved by a complete theory of the motive 

 power of heat, are the following : — 



I. Wliat is the precise nature of the thermal agency by means of 

 which mechanical effect is to be produced, without effects of any other 

 kind ] 



II. How may the amount of this thermal agency necessary for 

 performing a given quantity of work be estimated ? 



I, On the nature of Thermal Agency, considered as a Moti\ 

 Power. 



The whole theory rests on a principle generally admitted as an 

 axiom, which Carnot expresses in the following tei'ms : | — 



" In our demonstration, we tacitly assume that after a body has 

 experienced a certain number of transformations, if it be brought 

 identically to its primitive physical state as to density, temperature, 

 and molecular constitution, it must contain the same quantity of heat 

 as that which it initially possessed ; or, in other words, we suppose 

 that the quantities of heat lost by the body under one set of opera- 

 tions, are precisely compensated by those which are absorbed in the 

 others. This fact has never been doubted ; it has at first been ad- 



* Published in 1824, in a work entitled, " Keflexions sur la Puissance 5Io- 

 trice du Feu." by Mons. S. Carnot. An account of Carnot's theory is also pub- 

 lished in the Journal de VEcole Poly technique, vol. xiv., 1834, in a paper by 

 Mons. Clapeyron. 



t An account of the first part of a series of researches undertaken by Mons. 

 Regnault, by order of the late French Government, for ascertaining the various 

 physical data of importance in the theory of the steam-engine, has been recently 

 published in the Memoires de VInstitut, of which it constitutes the twenty-first 

 volume (1847). The second part of the researches has not yet been published. 



I The passage quoted in the text is translated from a note to p. 37, in Car- 

 not's Treatise. 



