4 



which the experiment is conducted." If such were the case, you and I have been 

 greatly mispending our time in searching by calculation for that which is given 

 by observation. 



" 8th. Not having by me the work of Daniell, to which you refer, I cannot ex- 

 amine the table to which you direct my attention. 



"I showed however several years ago, that many of his numerical results were 

 erroneous. (Dublin Philosoph. Journal, No. 1. Review of Daniell's Meteorolog. 

 Essays.) 



" I am, dear Sir, very truly yours, 

 "James Apjohn." 



In his Cth stitement he admits (above) that if I knew the temperature 

 of dry air, corresponding to any temperature of the hygrometer, I should be 

 in possession of a method for inferring the dew-point ; in his second letter he 

 had said that D must be determined for every possible temperature ; in his 

 fourth letter, again, he apparently relapses into his original error, saying 

 that "a mo'Sitelaborate series of experiments would be necessary, inasmuch as 

 the depression in dry air varies with the temperature of the wet ball. " His 

 fourth letter was as follows : 



(No. 4.) 



" My dear Sir, — I have been prevented by the illness of * * *, from attend- 

 ing to your letter of yesterday ; but in looking it over I do not find I have any- 

 thing to add to what I have already stated to you. I will, however, in conclusion, 

 repeat, that your numbers for the depression* of the hygrometer in dry air, are 

 greatly erroneous, and that a most elaborate series of experiments would be necessary 

 to render your hygrometrical process available, in as much as the depression in dry 

 air varies with the temperature of the wet bulb hygrometer. I have not been able 

 to look at the table to which you have referred me, but from your note just re- 

 ceived, this can be a matter of no importance. 



" J. Apjohn." 



The Letters marked 2, 3, and 4, had no dates, but they were all received 

 before the lOth of November, 1834. I then wrote the paper f which has 

 been published in the London and Edinburgh Fliiiosophical Magazine for Oc- 

 tober 1 835, and inclosed it to Dr. Apjohn with a letter, (2'tth November 1 834',) 

 asking him to bring it before the Academy for me. The Academy met on 

 the 29th November, and on the 30th Dr. Apjohn returned the paper with the 

 following letter : 



(No. 5.) " 28, Lower Baggot street. 



" November 30. 

 " My dear Sir, — I have been so much occupied that I have had scarcely time 

 to look over your paper with proper attention ; most of the topics however touched 

 upon in it, you have already been good enough to acquaint me with, and 1 be- 

 lieve I have ventured to observe upon them. The principle of your method I 

 have, I believe, on a former occasion stated to be, in my opinion, correct, for V : 

 V —y : : moisture of saturation, to actual moisture, is easily deducible from my 

 formula ; with the value of V, however, you do not appear to be acquainted, though 

 I trust you will not continue so long, but that you will investigate it by some of the 

 METHODS which you have suggested : until you have done this I think you had 

 better not submit your paper to the public, but you will, of course, in this par- 

 ticular, be altogether governed by your own judgement ; I trust you will also 

 apply yourself to the corrections, particularly that rendered necessary by the 



* The same mistake again. (See my 2nd note f, on Letter 1.) — H. H. 



+ The notes in pages 259 and 261, together with the numbers in the fifth column 

 of the Tabic, and from the words "the principal recommendations " in p. 263 to 

 the end of p. 264 were added to the paper in March 1835, subsequently to the 

 publication of Dr. Apjohn's first paper. 



