544 PROFESSOR WILLIAM THOMSON'S ACCOUNT OF 



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

 can be absorbed into a body without either elevating its temperature, or becoming 

 latent, and producing some alteration in its physical condition; and the fundamental 

 axiom adopted by Caexot may be considered as still the most probable basis for an 

 investigation of the motive power of heat ; although this, and with it every other 

 branch of the theory of heat may ultimately require to be reconstructed upon an- 

 other foundation, when our experimental data are more complete. On this un- 

 derstanding, and to avoid a repetition of doubts, I shall refer to (Jarnot'.s funda- 

 mental principle, in all that follows, as if its truth were thoroughly established. 



9. We are now led to the conclusion that the origin of motive power, de- 

 veloped by the alternate expansions and contractions of a body, must be found in 

 the agency of heat entering the body and leaving it ; since there cannot, at the 

 end of a complete cycle, when the body is restored to its primitive physical condi- 

 tion, have been any absolute absorption of heat, and consequently no conversion 

 of heat, or caloric, into mechanical effect ; and it remains for us to trace the pre- 

 cise nature of the circumstances under which heat must enter the body, and 

 afterwards leave it, so that mechanical effect may be produced. As an example, 

 we may consider that machine for obtaining motive power from heat with which 

 we are most familiar — the steam-engine. 



10. Here, we observe, that heat enters the machine from the furnace, through 

 the sides of the boiler, and that heat is continually abstracted by the water em- 

 ployed for keeping the condenser cool. According to Carnot's fundamental prin- 

 ciple, the quantity of heat thus discharged, during a complete revolution (or double 

 stroke) of the engine must be precisely equal to that which enters the water of 

 the boiler ;* provided the total mass of water and steam be invariable, and be re- 

 stored to its primitive physical condition (which will be the case rigorously, if the 

 condenser be kept cool by the external application of cold water, instead of by in- 

 jection, as is more usual in practice), and if the condensed water be restored to 

 the boiler at the end of each complete revolution. Thus, we perceive, that a cer- 

 tain quantity of heat is let down from a hot body, the metal of the boiler, to ano- 

 ther body at a lower temperature, the metal of the condenser ; and that there 

 results fi'om this transference of heat, a certain development of mechanical effect. 



11. If we examine any other case in which mechanical effect is obtained 

 from a thermal origin, by means of the alternate expansions and contractions of 

 any substance whatever, instead of the water of a steam-engine, we find that a 

 similar transference of heat is effected, and we may therefore answer the first 

 question proposed, in the following manner : — 



The thermal agency hy which, mechanical effect may be obtained^ is the trans- 

 fe,rence of heat from, one body to another at a lower temperature. 



• So generally is Carnot's principle tacitly admitted as an axiom, that its application in this 

 case has never, so far as I am aware, been questioned by practical engineers. 



