'Intelligence and Miscellaneous Articles. 477 



In conclusion Mr. Maw put forward some hypotheses as to the 

 period when the degradation of the older formations (the materials 

 of which compose the Drift) took place, the manner in which the 

 Drift was deposited, the extent of the submergence of England and 

 Wales during the period of its deposition, and the influence of 

 glaciers and glacier- action in its production. 



LXXII. Intelligence and Miscellaneous Articles. 



Otf THE DYNAMICAL THEORY OF HEAT. 



To John Tyndall, Esq., F.R.S., $c. 

 Sir, 

 f AGREE with the Editors of the Philosophical Magazine, that it 

 * is not desirable that my remarks on the dynamical theory should 

 give rise to prolonged discussion. What is wanted is experimental 

 proof of the direct conversion of heat into mechanical work ; and 

 after what has been so ably achieved by Dr. Joule for physical 

 science, in proving beyond a doubt the conversion of work into heat, 

 and determining the numerical equivalent of the transformation, 

 surely it is reasonable to expect that every effort should be made to 

 obtain positive physical evidence of the reality of the conversion of 

 heat into work. I thank the Editors for kindly allowing me to state 

 the difficulties which I have encountered in this pursuit, and the con- 

 sequent doubts which have presented themselves of the completeness 

 of the dynamical theory as it now stands ; and I beg to avail myself 

 further of their courtesy to clear up in a few lines some want of per- 

 spicuity in my last letter, which may in some degree have influenced 

 the remarks of Dr. Rankine and of Mr. Croll in the Philosophical 

 Magazine for March. 



Dr. Rankine is right in pointing out my inadvertence in overlook- 

 ing the application of the second law of the action of heat in two 

 instances. In the first, namely, my statement that the regenerator 

 might take up and restore all the heat remaining in air after it has 

 done work by expansion, my omission was but half unconscious, 

 and arose from my having uppermost in my mind the ideas of thermo- 

 dynamic action published by me some years ago ; and had I en- 

 tered more into detail on this point in my last letter it would have 

 been seen that my oversight was more in appearance than in reality, 

 as the difficulty of the hypothetical case proposed by me would not 

 have been removed by a formal application of the second law. In 

 the second case, the estimate I gave of the useful effect of steam- 

 engines was the old statement of M. Regnault ; and I should have 

 stated that it was quoted by me from M. Verdet's Second Lecture on 

 the Thermo-dynamical Theory of Heat (Paris, 15th February 1862, 

 section 2). Of course, in this case the application of the second 

 law made it requisite to suppose (in addition to M. Regnault's de- 

 ductions from the mere calculation of the latent heat) the conden- 

 sation of some steam as a result of the actual conversion of heat 

 into work ; but as this is the very point at issue, and a point on 

 which the grand question of the practical improvement of our heat 

 engines mainly depends, I may be allowed to state that, as far back 



