467 



the temperature of the air and wet bulb; p f the existing ; and 30, the 

 mean pressure ; m is a coefficient depending on the specific heat of 

 air and the caloric of elasticity of its included vapour, its arithmetical 

 value being -01149, or the equivalent vulgar fraction ^ which he 

 afterwards says he is " disposed to consider as more correctly repre- 

 sented by the fraction g 1 ^," but has not been able fully to satisfy him- 

 self ("Phil. Mag." vii., 472). In constructing the following Tables, I 

 have used the fraction ^ as coefficient. 



The other formula is that of Professor August, of Berlin, modified 

 by Begnault (" Poggendorf Annal.," vol. v., 2 series, p. 69) : 



1 0.429(*-Q 

 X ~ J 610 ■ 



in which x is the elastic force of aqueous vapour actually in the air ; / 

 and /' are the elastic forces of the saturated vapour of water for the 

 temperatures t and V ; t is the temperature of the air and dry bulb ; t' f 

 the temperature of the wet bulb ; h, the height of the barometer ; 

 610 — t' is the latent heat of the vapour of the water; the coefficient 

 0*429 requires occasional variation, as will be seen hereafter. 



The following Table contains a few fractions of saturation calculated 

 byApjohn's formula, from his test experiments, using ^ as thecoefficient ; 

 in deducing them I have employed the elasticities of vapour given in 

 Regnault's Tables, as interpreted and corrected for the latitude of Dublin 

 by the Pev. P. Y. Dixon. I have placed beside them the fractions of 

 saturation, calculated by the modified formula of August, the French 

 temperatures and pressures being converted into English equivalents : — 



Table I. 



Dry Bulb, 

 Fahr. 



Wet Bulb, 

 Fahr. 



Barometer. 



Fraction of Satura- 

 tion by Apjohn's 

 Formula, derived 

 from Test Experi- 

 ments. Coefficient 

 88. 



Fraction of Satura- 

 tion by August's 

 Formula^ modified 

 by Regnault's, 

 Coefficient '429. 



Differences. 



o 



78 



o 



62'2 



30-30 



0-395 



0-383 



0 



•012 



90 5 



67 



30-15 



0-274 



0 271 



0 



•003 



68 



60-3 



30-42 



0 635 



0627 



0 



•008 



72 



62 



30-51 



0-561 



0 -552 



0 



•009 



98-5 



71-5 



30-36 



0-251 



0-240 



0 



•011 



77 



65 



30-51 



0-515 



0-507 



0 



•008 



69 



58-6 



30-30 



0527 



0-522 



0 



•005 



72 



60 



30-30 



0-485 



0-474 



0 



•011 



92 



69 



30-42 



0-295 



0-288 



0 



•007 



