Wednesday the 29th of October I wrote to Dr. Apjolin, (with wliom I was 

 not previously acquainted,) communicating the formula my views had led 

 me to adopt. His reply of the follovving evening was in these terms : 



(No. 1.) 



"28, Lower Bag got Street, 

 " Tlinrsday Evening. (30th October, 1834.) 



" My dear Sir, — I feel much flattered by the observations which you have 

 been pleased to make upon my paper read at the Academy, because they obviously 

 proceed from a person who has devoted attention to the subject discussed in it, 

 I have also read with much pleasure the views which have occurred to you in re- 

 ference to the same question, and as I am sure you have communicated them to 

 me in a scientific spirit, I shall not hesitate to state to you the opinion which I 

 entertain respecting them. Your formula for expressing the amount of caloric 

 necessary for the formation of a given weight * of vapour at any temperature i 

 is perfectly correct ; my impressions however are different in reference to the next 

 topic which you discuss, namely, the temperature of dry air corresponding to any 

 temperature of the wet bulb hygrometer, thus at temperatures indicated by the 

 hygrometer of 92°, 65°, and 32°.f You state the corresponding temperatures of 

 dry air to be 155°, 94°, and 42°-4 ; whereas by my formula they would be 215°-28, 

 1 18°-59, and 49°-4 ; but supposing the numbers in your table to be correct, 

 I am disposed to think that your method, the conception of which is certainly 

 highly ingenious, would give at least a good approximation to the dew-point. 

 I have now only to observe upon your concluding formula. In it, I think you 

 will find that there is some error, for upon making the calculation I find that the 

 density of vapour at 65° is not (as you state it,) -0163, but '026. 



" In conclusion, I beg to state, that I shall be most happy to communicate with 

 you further, should you be inclined, on this subject, and that 



"I am, very sincerely yours, 

 "James Apjoiin, 



i< N.B. I take advantage of the circumstance of this not being sent you as early 



as I intended it should, to retract to a certain extent an observation already made. 

 [ have said, that the principle of your method for collecting the amount of mois- 

 ture in air, from the temperature of the wet bulb hygrometer, appeared to me 

 sufliciently correct to afford a good approximation. Upon consideration, how- 

 aver, I fear that this is not the case; You assume that the depression indicated 

 by the hygiometer, in dry air, is to the depression in air containing moisture, as 

 the satur'atmg amount of vapour at the temperature of the hygrometer to the dif- 

 ference between this quantity and what is actually present. Such proportion 

 would be| true of two portions of air at the same temperature, the one dry and 

 the other moist, but I believe in no other case." 



(No 2.) 



In reply I sent him the proofs of my formulae (see Lond. and Edinb. Phi- 

 losophical Magazine, vol. vii. p. 257,) and his answer was as follows: 



" My i,EARS,iR,—Bothyourformula,—ih!ittorihe quantity of caloric inagiven 

 bulk of vapour at any temperature, t, and that for the density of vapour,— are <p,ite 

 correct in reference to the standardwhich you adopt. In the latter formula you assume 



* This should be volume; he has corrected it in letter 2. — H. H. 



t These were only taken hypothetically, thus : " If dry air at 94° makes hy- 

 jrroraeter fall to 65°,' then the corresponding temperatures of dry air when the hy- 

 grometer is at 92" and 32° will be 155° and 42<'-4 respectively. " It was merely 

 an exemplification of the use of the proportional numbers in my table.— H. H. 



+ This is quite erroneous. The proportion would be impossible if dry and 

 moist air of the same temperature were to act on the hygrometer, for the stationary 

 temperature of the latter would be very different in the two cases, and ot course 

 there could be no comparison. But it is true, when dry and moist uir are of such 

 different temperatures, that they cause the wet bulb to fall to the same temperature 

 in each case. — H. II. 



