56 H. F. Blanford — Experiments with the Psychrometer. [No. 2, 



point, f the same at the temperature t' of the wet bulb, t the air tempera- 

 ture and h the barometer reading. 



The development of this formula may be found in Regnault's original 

 paper, published in the Comptes rendus for April 1845, or in the transla- 

 tion given in the 3rd Volume of Taylor's Scientific Memoirs. Also in the 

 article ' Hygrometry' in Watts' s Chemical dictionary. It is based on the 

 assumption that the film of air around the wet bulb is saturated with 

 vapour, and that the heat lost by this film of air, in falling to the tempera- 

 ture of the wet bulb, is exactly equal to the latent heat absorbed by the 

 water which passes into vapour in the act of bringing it to saturation. 



The second is Apjohn's well-known formula, given in almost all English 

 manuals of physics and meteorology, as follows, for temperatures of f 

 above the freezing point : 



- t—t' h 



so = / . — 



J 88 30 



The development of this formula was given by Dr. Apjohn in the 

 Transactions of the Eoyal Irish Academy, November 1834, and is reproduced in 

 Professor Everett's translation of Deschanel's Natural Philosophy. It pro- 

 ceeds on precisely the same assumption as the previous formula, but assumes 

 a mean constant instead of a variable value for the latent heat of vapour, 

 by which the formula is somewhat simplified ; the difference of the results 

 afforded by Apjohn's and August's formula depends, however, mainly on 

 the different values assumed for the constant coefficients common to the 

 two formula?. 



In applying these formulae, I have taken the vapour-tensions from a 

 table lately computed for the mean latitude of 22°, from that given by the 

 Rev. Pobert Dixon for the latitude of Dublin. 



Glaisher's factors, with which the third values of the dew-point are 

 obtained, are those published in 1856. Their use is of course open to the 

 objection that they take no account of variations of barometric pressure. 

 As a rule they seem to give a result too low with a high relative humidity 

 and too high with low humidities. 



