138 THOMSON ON CARNOT'S 



mechanical effect; and, consequently, engines may 

 be constructed in which the whole or any portion 



tively[demands an answer to this question ; yet no answer 

 can be given in the present state of science. A few years 

 ago, a similar confession must have been made with refer- 

 ence to the mechanical effect lost in a fluid set in motion in 

 the interior of a rigid closed vessel, and allowed to come to 

 rest by its own internal friction; but in this case the 

 foundation of a solution of the difficulty has been ac- 

 tually found in Mr. Joule's discovery of the generation 

 of heat, by the internal friction of a fluid in motion. En- 

 couraged by this example, we may hope that the very per- 

 plexing question in the theory of heat, by which we are 

 at present arrested, will before long be cleared up. 

 [Note of Sept., 1881. The Theory of the Dissipation of 

 Energy completely answers this question and removes the 

 difficulty.] 



It might appear that the difficulty would be entirely 

 avoided by abandoning Carnot's fundamental axiom ; a 

 view which is strongly urged by Mr. Joule (at the conclu- 

 sion of his paper " On the Changes of Temperature pro- 

 duced by the Rarefaction and Condensation of Air." Phil. 

 Mag., May 1845, vol. xxvi.) If we do so, however, we 

 meet with innumerable other difficulties insuperable 

 without farther experimental investigation, and an entire 

 reconstruction of the theory of heat from its foundation. 

 It is in reality to experiment that we must look either 

 for a verification of Carnot's axiom, and an explanation of 

 the difficulty we have been considering; or for an entirely 

 new basis of the Theory of Heat. 



