MOTIVE POWER OF HEAT. 137 



11. On the measurement of Thermal Agency, 

 considered with reference to its equivalent of 

 mechanical effect. 



12. A perfect thermodynamic engine of any 

 kind is a machine by means of which the greatest 

 possible amount of mechanical effect can be obtained 

 from a given thermal agency; and, therefore, if in 

 any manner we can construct or imagine a perfect 

 engine which may be applied for the transference 

 of a given quantity of heat from a body at any 

 given temperature to another body at a lower given 

 temperature, and if we can evaluate the mechanical 

 effect thus obtained, we shall be able to answer 

 the question at present under consideration, and 

 so to complete the theory of the motive power 

 of heat. But whatever kind of engine we may 

 consider with this view, it will be necessary for us 

 to prove that it is a perfect engine; since the 

 transference of the heat from one body to the other 

 may be wholly, or partially, effected by conduction 

 through a solid,* without the development of 



*When " thermal agency" is thus spent in conducting 

 heat through a solid, what becomes of the mechanical 

 effect which it might produce? Nothing can be lost in 

 the operations of nature no energy can be destroyed. 

 What effect, then, is produced in place of the mechanical 

 effect which is lost ? A perfect theory of heat impera- 



