as deduced by Jiimselfand Dr. Suerman. 345 



thrust forward, and the wet bulb thus permanently subjected 

 to the action of a gaseous current in a state of perfect dry- 

 ness. 



Having stated this, which I consider as the gravest objec- 

 tion to which the experiments of Suerman are exposed, I shall 

 next advert to, for the purpose of correcting, a couple of mis- 

 takes into which he has fallen in reference to my researches 

 upon the subject of specific heats. 



In the first place then Suerman, p. 81, seems to think that 

 in my computations I have considered the caloric of the ela- 

 sticity of vapour as an invariable quantity, whereas in point of 

 fact it is the sum of it and the sensible heat of vapour whose 

 value theory and experiment would seem to concur in proving 

 to be constant. This is a misapprehension. In ray first paper 

 on the dew-point, p. 4, will be found the following sentence: 

 " In strictness the number employed (to represent c, the 

 latent heat of vapour) should be 967 + 212— t, but it would 

 be easy to show that the uniform use of 1129 (the value of e 

 at 50°) cannot give rise to any material error." The latter 

 part of this sentence was intended to apply solely to the me- 

 teorological use of my formula, and not at all to it when em- 

 ployed in investigating the question of gaseous specific heat. 

 Dr. Suerman has fallen into this misconception from the cir- 

 cumstance of his having seen only the abridged account of 

 my first series of researches on this subject as published in 

 the Reports of the British Association for the Promotion of 

 Science. Had he however repeated the calculation of any one 

 of my experiments, he would have seen that in the case of the 

 specific heats I had adopted what he considers as the rigorous 

 method of estimating the value of e, although I set down the 

 sum of the sensible and latent heat of steam at every tempera- 

 ture as somewhat less than was done by him. 



In reference to the second point, it is only necessary for 

 me to recall to the attention of the reader the explanatory 

 statement which I have already made. 



From the last table given above it will be seen that, if we 

 except oxygen, Suerman's results and mine are almost co- 

 incident. This correspondence has been noticed and admitted 

 by him, as is obvious from the following passage : " Siquidem 

 ad diversissimum attendamus apparatum quo usus est, faten- 

 dum: satis bene illis convenire experimentaD. Apjohn atque 

 nostra. Utraque vero multum distant ab experimentis De 

 la Roche atque Berard, ad quae nostra propius accedunt, ra- 

 tione fluidorum elasticorum elementariorum, experimenta D. 

 Apjohn ratione aerum compositorum." The opinion here 

 expressed cannot be considered as very well sustained by the 



