Prof. Tyndall on Mayer, and the Mechanical Theory of Heat. 173 



state of things in which the ice has dwindled to limits which 

 barely serve as a key to the stupendous operations of a by-gone 

 geologic age. To account for a glacial epoch, then, we need not 

 resort to the hard hypothesis of a change in the amount of solar 

 emission, or of a change in the temperature of space traversed 

 by our system. Elevations of the land, which would naturally 

 accompany the gradual cooling of the earth, are quite competent 

 to account for such an epoch ; and the ice itself, in the absence 

 of any other agency, would be competent to destroy the condi- 

 tions which gave it birth. 



Royal Institution, August 18, 1862. 



XXIV. Mayer, and the Mechanical Theory of Heat. 

 By Professor Tyndall, F.R.S. 



My dear Joule, 



ON my return from Switzerland two days ago, I became ac- 

 quainted with the note which you have published in the 

 last Number of the Philosophical Magazine. Would you allow 

 me to make the following remarks in connexion with the subject 

 of it? 



During the spring of the present year I gave at the Royal 

 Institution a course of lectures (t On Heat, regarded as a kind 

 of Motion." During the early portion of the course, I had en- 

 gaged a short-hand writer to report the lectures, with a view to 

 their subsequent publication ; and from this gentleman's notes 

 of my second lecture I make the following extract, which refers 

 to the mechanical theory of heat : — " It is to Mr. Joule, of 

 Manchester, that we are almost wholly indebted for the experi- 

 mental treatment of this subject. With his mind firmly 

 fixed upon a principle, and undismayed by the coolness with 

 which his first labours appear to have been received, he per- 

 sisted for years in his attempts to prove the invariability of the 

 relation between heat and ordinary mechanical force. He placed 

 water in a suitable vessel, agitated it by paddles moved by 

 measurable forces, and determined the elevation of temperature ; 

 he did the same with mercury and sperm- oil. He also caused 

 disks of cast iron to rotate against each other, and measured the 

 heat produced by their friction. He urged water through 

 capillary tubes, and measured the heat thus generated. The 

 results of his experiments leave no doubt upon the mind that 

 under all circumstances the absolute amount of heat produced 

 by the expenditure of a definite amount of mechanical force is 

 fixed and invariable." Such has been my language regarding 

 you; and to it I still adhere. I trust you find nothing in it which 

 indicates a desire on my part to question your claim to the honour 



