air at 212'* to be represented by unit)', upon which hypothesis "625 is the specific 

 gravity of steam at 212°, and •0163 its specific gravitj- at 65". You must be 

 aware, however, that these numbers are not comparable with those in our tables, 

 in as much as they are related to a different radix. My calculation of the density 

 of vapour at 63° ( by the way it is 'Ol 2r and not "026, as by mistake I wrote it,) 

 is conducted upon the usual supposition, of air at 60° being the unit; your result 

 however beoraes the same with mine, by applying the proper correction, or by 

 diminishing it in the ratio of 660 : 508. The general expression, in fact, for the 

 specific gravity of the maximum amount of vapour corresponding to any tem- 

 perature, is "625 X ~^ X 448 ^f ,/ being the elastic force of vapour, and t its tem- 

 perature. 



"I have now. Sir, much pleasureininformingyou that your method* of inferring 

 the exact amount of moisture at any time existing in air, is perfectly correct. I 

 have deduced from my own method, that (D being the reduction of temperature 

 experienced by the hygrometer in dr\- air, and d the reduction in moist) D : D — d 

 '•'■/'■ /"'< f being the elastic force of vapour at the temperature of the hygrometer, 

 and/" at dew-point, a conclusion equivalent to yours ^. Your method, however, 

 cannot admit of practical application until D be determined for every possible 

 temperature of the hygrometer. This is a thing which I can do theorHically ; and 

 as soon as I have leisure, I shall be happy to furnish you w^ith such a table. 

 I should, however, be most happy if you would undertake it experimentally. Even 

 a few experimental results would be highly interesting. 



" Believe me, dear Sir, very sincerely yours, 

 "James Apjohx." 



= *' It is scarcely necessary to say th*it this is but an approximate conclusion and 

 that it will require the application of the corrections which I have described in 

 my paper. — J. A." 



I replied to this : but some expression in my letter appears to have annoyed 

 Dr. Apjohn, judging from his answer, which follows : 



(No. 3.) 



"My Dear Sir, — In reply to your note just received, I beg to make the fol- 

 lowing statements : — 



" 1st. In my answer to your first communication, I stated that your formula for 

 the amount of heat in a given volume of vapour, was correct, but that your value 

 of the density of vapour at 65° was otherwise. These statements I repeated in 

 my second note to you, so that, as far as respects them, I cannot perceive the in- 

 sistency which you seem to glance at. 



" 2nd. I differ from you as to the number of corrections to be applied, and as 

 to the possibility of — consistently with rigid accuracy — omitting any of them. 



" 3rd. I do not agree with you as to the effect of radiation on the wet bulb hy- 

 grometer, though I am not aware of any accurate method of correcting for it. 



" 4th. I am quite satisfied that your tablef of the temperatures of the hygro- 

 meter and dry air is quite incorrect. 



" 5th. That such being the case, your value of D is erroneous, and so also must 

 any conclusion be deduced from it. 



"6th. I beg AGAIN to observe, that as soon as you can tell the temperature of dry 

 air corresponding to any temperature of the hygrometer, and be able to apply the 

 necessary corrections, you will be in possession of a method for inferrimj the dew- 

 point, by means of the wet bulb hygrometirr. 



" 7lh. I am quite at a loss to understand what you mean by saying that "the 

 temperature of the wet bulb hygrometer, j«+ the dew-point of the atmosphere in 



• Comparing the mistake pointed out in the postscript to his first letter, with 

 the deduction here subsequently arrived at, I think it will be obvious to any 

 one, that the very idea of the action of dry air having anything to do with the 

 formula had not previously occurred to Dr. Apjohn at all, and of course that the 

 experimental method is mine. — H. H. 



+ This is the same mistake pointed out in the 2nd note on his first letter. — II. II. 



4 For " is " read "t/iics. "— H. H. 



