heat communicated to the hygrometer by radiation, for without such your me- 

 thod would afford but ari approximate result. With respect to introducing 

 your paper to the Academy, this I shall certainly do at the next or any subsequent 

 meeting, should it be your wish ; 1 thmk however that, being a member yourself, I 

 could not with propriety take the subject out of your hands. 



" Believe me, dear Sir, very truly yours, 



"James Apjohn." 

 "N.B. In reference toyour postscript,! beg to say that I of course represent by 

 /'and/" the maximum elasticities of vapour at the temperatures of the hygrome- 

 ter and dew-point ; these elasticities, by the way, are not strictly as the saturating 

 quantities of moisture at those temperatures ; but the error is altogether inappre- 

 ciable, and is in my method allowed for. — J. A." 



The methods which Dr. Apjohn here so distinctly admits to have been svg- 

 gestedby me in that paper, were these: — One was to expose a wet and dry 

 thermometer (equally) to a current of air dried artificially and heated at 

 pleasure. This Method Dr. Apjohn adopted in March 1835, and published 

 as his own. The other method 1 pointed out was to use conjoint observations 

 of the " dew-point, " the " temperature of the wet ball," and the " tempe- 

 rature of the air ;" and the best mode of investigating by this method was 

 stated by me to be similar to the former method, only using common (not 

 dried) atmospheric air, heated artificially. This method Dr. Apjohn adopted 

 in February 1835, and subsequently published without acknowledgment. 

 I also stated in my paper that these latter experiments would be best tried 

 when the atmosphere was perfectly damp, as the knowledge of the dew-point 

 would then be perfect, and Dr. Apjohn's experiments (in which the air was 

 rendered perfectly damp artificially before heating it,)were commenced about 

 the middle of April 183.5. (See his 2nd paper.) 



Dr. Apjohn having advised me in the above letternot to bring my paper 

 before the public, until I had made the experiments suggested in it, I laid 

 it by, intending, when I should have leisure and could procure the necessary 

 apparatus, to make those experiments. Feeling grateful for what I con- 

 sidered _7«(/icJoM5 and friendly advice, 1 instituted (in compliance with Dr. Ap- 

 john's wishes in his letters, Nos. 3 and 5,) some additional experiments on 

 the radiation of heat as immediately connected with the wet ball hygrometer, 

 (see 5th Report of British Association, p. 165. Experiments 3 and 4,) and 

 communicated them to Dr. Apjohn in February 1835. They apparently 

 satisfied him that no error could arise from this cause. Early in March I 

 learned from him, incidentally, that he had adopted my methods of experi- 

 menting, and being convinced that my paper would lose any little interest 

 which it mighthave, if not published before the experiments, I sent it,about the 

 middle of March 1835, to the London and Edinburgh Philosophical Magazine, 

 mentioning in a note that Dr. Apjohn had adopted those methods, and having 

 no idea that he would not have acknowledged my communications on the 

 subject. 



Ur. Apjohn read his 2nd paper to the Academy on the 27th of April 1835, 

 containing his experimental results,* and transmitted it to your Magazine 

 without any acknowledgment of my having suggested such experiments 

 to him : under these circumstances I felt myself justified in laying claim to 

 " having suggested those methods of experimenting to Dr. Apjohn." 



This claim having been alleged by Dr. Apjohn in the Academy to be wholly 

 without foundation, I felt it to be most respectful to the Academy to propose 

 that the correspondence between us should be submitted, either to the Council 

 of the Academy, or to a committee, to be appointed by them to inquire into 

 the subject anil report thereon to the Academy. I stated that I would submit 

 to them Dr. Apjohn's letters to me, and uikcd Dr. Apjohn to lay my letters 



