Prof. Clausius on the Mechanical Theory of Heat. 463 



the particular path by which the observer may pass from one of 

 those points to the other. But if the directions of the lines at 

 A and B can be conceived as absolutely identical, the lines will 

 also be in the same direction with each other at any other points 

 C and D, inasmuch as each straight line is everywhere in the 

 same direction. 



LXI. Reply to a Note of Mr. Joule, contained in the November 

 Number of the Philosophical Magazine. By Prof. Clausius*. 



IN the November Number of this Journal a note appears from 

 Mr. Joule, which refers to a passage in my memoir " On the 

 Application of the Mechanical Theory of Heat to the Steam- 

 engine,^^ where the experiments conducted by Mr. Joule and 

 Mr. Thomson are mentioned. The note concludes with the fol- 

 lowing words : — "The results we have arrived at in our joint in- 

 vestigation have tended certainly to develope our views, and to 

 give a more definite knowledge of the constitution of elastic fluids, 

 but they do not contradict our original statements, published 

 before the appearance of Prof. Clausius^s papers.^' I can only 

 explain this note, and particularly the conclusion of it, so far as 

 Mr. Joule himself is concerned, by referring it to a misunder- 

 standing of my meaning. But as I believe that I myself, by an 

 inaccuracy of expression, have given rise to this misunderstanding, 

 I feel it to be my duty to say a few words here in explanation. 

 I believe that the note has been chiefly called forth by the fol- 

 lowing words in my memoir, which precede the reference to the 

 experiments mentioned : — " Even the physicists which had occu- 

 pied themselves more especially with the mechanical theory of 

 heat did not at that time (1850) coincide with this view of the 

 deportment of vapour.^* As Mr. Joule, so far as I know, has 

 nowhere published his views on the deportment of vapour, these 

 words could not of course refer to him ; but I ought perhaps, in 

 order to avoid such a misunderstanding, have expressly limited 

 the statement to such physicists as had applied the mechanical 

 theory of heat to vapours at their maximum density. 



With respect, further, to the deportment of gases, which, in 

 the portion of my memoir already referred to, is brought into 

 connexion with the deportment of vapours, I never entertained 

 the least doubt that Mr. Joule, in his beautiful investigations 

 " On the Changes of Temperature produced by the Rarefaction 

 and Condensation of Air,^^ had started from a perfectly correct 

 point of view, and that the results of the new experiments above 

 mentioned do not contradict his former statements, but, on the 

 contrary, have served to confirm them in all essential particulars. 



Zurich, Nov. 16, 1856. 



* Communicated by the Author. 



