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; andthe fundamental 
axiom adopted by Carnot 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 Carnot’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 from 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 by which mechanical effect may be obtained, is the trans- 
Jerence of heat from one body to another at a lower temperature. 
* So generally is Carnor’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. 

