Humidity of tlie Air. 171 



Art. XIII. — Remarks on Tables for calculating tlie Humidity of 

 the Air, ly H. C. Russell, Esq., B.A. 



[Bead before tlie Society, Deer. 8th, 1869.] 



Before proceeding to explain my humidity curves, I will compare 

 the work necessary, in order to find the humidity by the two 

 methods in common use. 



I need scarcely remind the members of this Society that the 

 most common method of finding the humidity of the air is by the 

 formula of Dr. Apjohn, of Dublin, or some modification of it. 

 For the purpose of the present comparison, I will make use of 

 his well-known formula, omitting only the small correction for 

 height of barometer ; and I do so because the other method does 

 not take the barometer into account. 



E 

 The formula is then : Humidity = — 



W 

 where N=Tension of vapour at dry bulb 



F=P' _ (01147xd) 

 P'=Tension of vapour at wet bulb 

 d= Difference between dry and wet. 

 Taking an example, the computation is — 



Dry bulb ... 63.3 .01147 



Wet ditto ... 55.7 . 7.6 



Difference ... 7.6 6882 



8029 



•087172 

 From table of vapour tension P' = .444 



.087 Humidity 



From table of vapour tension N=.582 .3570 .613 



3492 or 61.3 



780 

 582 



1980 

 1746 



The other method coming, I believe, into very general use is 

 by the tables of Mr. G-laisher, of the Royal Observatory, 

 Greenwich. 



