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XXXVIL—An Account of Carnot’s Theory of the Motive Power of Heat ;* with 
Numerical Results deduced from Ruenavyt’s Experiments on Steam.t By 
Witu1am Tuomson, 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 currents 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, <‘ Réflexions sur la Puissance Motrice du Feu, et sur 
les Machines Propres 4 Déveloper cette Puissance. Par S. Carnot.” An account of Carnot’s Theory is 
also published in the Journal d’ Ecole Polytechnique, vol. xiv., 1834, in a paper by Mons. Crareyron. 
+ An account of the first part of a series of researches undertaken by Mons. Recnavtt, 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 Expériences,”’ 
&c.) in the Mémoires de V Institut, of which it constitutes the twenty-first volume (1847). The 
second part of these researches has not yet been published. 
VOL. XVI. PART V. C& 
