200 



is generated by the friction of fluids in motion seem to overturn the 

 opinion commonly held that heat cannot be generated, but only pro- 

 duced from a source where it has previously existed, either in a 

 sensible or in a latent condition. 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 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 ' it, 

 may ultimately require to be reconstructed on another foundation, 

 when our experimental data are more complete. On this understand- 

 ing the author of the present paper refers to Carnot' s fundamental 

 principle, as if its truth were thoroughly established. 



If we consider any case in which mechanical effect is obtained from 

 a thermal origin, by means of the alternate expansions and contrac- 

 tions of any substance whatever, and follow a perfectly rigorous pro- 

 cess of reasoning indicated by Carnot, we arrive at the following 

 conclusion, by which the first proposed question is answered : — 



The thermal agency hy which mechanical effect may be obtained, is 

 the transference of heat from one body to another at a lower tem- 

 perature. 



II. On the measurement of Thermal Agency, considered with re- 

 ference to its equivalent of mechanical effect. 



The criterion of what may be called a perfect thermo-dynamic 

 engine is thus stated : — 



A perfect thermo-dynamic engine is such, that, whatever amount 

 of mechanical effect it can derive from a given thermal agency, if 

 an equal amount be spent in working it backwaxds, an equal reverse 

 thermal effect will be produced. 



Any two perfect engines, however different in their constructions, 



conductor. I am not aware that any examination, with reference to the truth 

 of this conjecture, has been instituted ; but in the case when the inducing body 

 is a pure electro-magnet (without any iron) the experiments actually performed 

 by Mr Joule render the conclusion probable, that the heat evolved in the wire 

 of the electro-magnet is not affected by the inductive action otherwise than 

 through the reflected influence, which diminishes the strength of its own cur- 

 rent. 



