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XXXVI. — An Account of Carnot's Theory of the Motive Power of Heat* with 

 Numerical Results deduced from Regnault's Experiments on Steam.-f By 

 William Thomson. Professor of Natural Philosophy in the University of 

 Glasgow. 



(Read January 2, 1849.) 



1. The presence of heat may be recognised in every natural object ; and there 

 is scarcely an operation in nature which is not more or less affected by its all- 

 pervading influence. An evolution and subsequent absorption of heat generally 

 give rise to a variety of effects ; among which may be enumerated, chemical 

 combinations or decompositions ; the fusion of solid substances ; the vaporisation 

 of solids or liquids ; alterations in the dimensions of bodies, or in the statical 

 pressure by which their dimensions may be modified ; mechanical resistance over- 

 come ; electrical cun-ents generated. In many of the actual phenomena of na- 

 ture, several or all of these effects are produced together ; and their complication 

 will, if we attempt to trace the agency of heat in producing any individual effect, 

 give rise to much perplexity. It will, therefore, be desirable, in laying the foun- 

 dation of a physical theory of any of the effects of heat, to discover or to imagine 

 phenomena free from all such complication, and depending on a definite thermal 

 agency ; in which the relation between the cause and effect, traced through the 

 medium of certain simple operations, may be clearly appreciated. Thus it is 

 that Carnot, in accordance with the strictest principles of philosophy, enters upon 

 the investigation of the theory of the motive power of heat. 



2. The sole effect to be contemplated in investigating the motive power of 

 heat is resistance overcome, or, as it is frequently called, " work performed,'" or 

 " mechanical effect^ The questions to be resolved by a complete theory of the 

 subject are the following : 



(1.) What is the precise nature of the thermal agency by means of which 

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



* Published in 1824, in a work entitled, "Reflexions sur la Puissance Motrice du Feu, et sur 

 les Machines Propres a Developer cette Puissance. Par S. Carnot." An account of Carnot's Theory is 

 aiso fiMishei in the Joiirnal d^ Ecole Poly technique, vol. xiv., 1834, in a paper by Mons. Clapeykon. 



t An account of the first part of a series of researches undertaken by Slons. Begnault, 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 (under the title, " Relation des Experiences," 

 &c.) in the Memoires de VInstitut, of which it constitutes the twenty-first volume (1847). The 

 second part of these researches has not yet been published. 



VOL. XVI. PART V. 7 A 



