I)r. W. J. M. Rankine on Thermodynamics, 291 



current of additional water along with it ; also, to a certain ex- 

 tent, the ejector-condenser." 



6. Retardation by Conduction. — The conduction of heat from 

 the parts of a stream where the pressure and temperature are 

 highest to the parts of the same stream where the pressure and 

 temperature are lowest, produces, according to the foregoing 

 principles, a gradual and permanent retardation of the stream, 

 independently of the agency of friction ; and this is accompanied 

 by the production of heat to an amount equivalent to the lost 

 energy of flow. 



XXXIV. On Thermodynamics. 

 By W. J. Macquorn Rankine, C.E., LL.D., F.R.SS.L. $ E. 



To the Editors of the Philosophical Magazine and Journal. 

 : Gentlemen, 



THE Rev. J. M. Heath, in his paper "On Thermodynamics," 

 published in the Philosophical Magazine for September, 

 states, with reference to certain dynamical principles, that I have 

 admitted, by my silence, that those principles were ignored by 

 the earliest original investigators in the science. This is a mis- 

 conception of the meaning of the statement made by me in my 

 letter published in your August Number, which was as fol- 

 lows :-■ — "This very principle" (that is, the principle that the 

 work done by a force in overcoming attractions and repulsions 

 cannot take effect in accelerating molecular motions) "has been 

 most carefully kept in view by every author of original researches 

 in thermodynamics, and by every writer on the subject who has 

 understood those researches." I did not keep silence respecting 

 the earliest original investigators ; for my statement obviously 

 comprehends every original investigator, early or late. The only 

 writers as to whom I kept silence were those who had not un- 

 derstood the original researches. 



Mr. Heath then refers to the more modern writers having 

 " altered their creed and language " in reference to the principle 

 just referred to. As regards this, I have to state that, so far 

 as my knowledge of the writings of original investigators and of 

 those who have understood the results of their researches extends, 

 there has been no such alteration in the course of the twenty-one 

 years which have elapsed since the mathematical principles of 

 thermodynamics were first set forth in a complete and systematic 

 form. The notation and the methods of demonstration have 

 been simplified, and the principles have been tested by new ex- 

 periments and applied to new problems in theory and practice ; 



