Cambridge Philosophical Society. 315 



theory of the motive power of heat enables us to give an affirmative 

 answer. 



The relation between motive power and heat, as established by 

 Carnot, is such that quantities of heat, and intervals of temperature, are 

 involved as the sole elements in the expression for the amount of me- 

 chanical effect to be obtained through the agency of heat ; and since 

 we have, independently, a definite system for the measurement of 

 quantities of heat, we are thus furnished with a measure for intervals 

 according to which absolute differences of temperature may be esti- 

 mated. To make this intelligible, a few words in explanation of 

 Carnot's theory must be given ; but for a full account of this most 

 valuable contribution to physical science, the reader is referred to 

 either of the works mentioned above (the original treatise by Car- 

 not, and Clapeyron's paper on the same subject). 



In the present state of science no operation is known by which heat 

 can be absorbed, without either elevating the temperature of matter, 

 or becoming latent and producing some alteration in the physical 

 condition of the body into which it is absorbed ; and the conversion 

 of heat (or caloric) into mechanical effect is probably impossible*, 

 certainly undiscovered. In actual engines for obtaining mechanical 

 effect through the agency of heat, we must consequently look for the 

 source of power, not in any absorption and conversion, but merely 

 in a transmission of heat. Now Carnot, starting from universally 

 acknowledged physical principles, demonstrates that it is by the 

 letting down of heat from a hot body to a cold body, through the 

 medium of an engine (a steam-engine, or an air-engine for instance), 

 that mechanical effect is to be obtained ; and conversely, he proves 

 that the same amount of heat may, by the expenditure of an equal 

 amount of labouring force, be raised from the cold to the hot body 

 (the engine being in this case worked backwards) ; just as mechanical 

 effect may be obtained by the descent of water let down by a water- 

 wheel, and by spending labouring force in turning the wheel back- 

 wards, or in working a pump, water may be elevated to a higher 

 level. The amount of mechanical effect to be obtained by the trans- 

 mission of a given quantity of heat, through the medium of any kind 

 of engine in which the ceconomy is perfect, will depend, as Carnot 

 demonstrates, not on the specific nature of the substance employed 

 as the medium of transmission of heat in the engine, but solely on 

 the interval between the temperatures of the two bodies between 

 which the heat is transferred. 



Carnot examines in detail the ideal construction of an air-engine 

 and of a steam-engine, in which, besides the condition of perfect 



* This opinion seems to be nearly universally held by those who have 

 written on the subject. A contrary opinion however has been advocated 

 by Mr. Joule of Manchester; some very remarkable discoveries which he 

 has made with reference to the generation of heat by the friction of fluids 

 in motion, and some known experiments with magneto-electric machines, 

 3eeming to indicate an actual conversion of mechanical effect into caloric. 

 No experiment however is adduced in which the converse operation is exhi- 

 bited; but it must be confessed that as yet much is involved in mystery 

 with reference to these fundamental questions of natural philosophy. 



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