374 S. A, Hill — PsycJirometer and Goiidensing Hijgrometer. [No. 4, 



becomes completely saturated with moisture. These assumptions lead to 

 the formula. 



f = f — p , in which the symbols have the following mean- 



/ = pressure of water vapour actually present in the air ; 



/' = pressure of saturated vapour at the temperature t^ ; 



t = temperature of air ; 



t' = temperature of wet bulb ; 



h = height of the barometer ; 



5 = specific heat of air under the actual conditions ; 



d = density of vapour compared to the actual air if the pressures 

 and temperatures were equal ; 



L = Latent heat of evaporation at the temperature t\ 



Dr. Apjohn, about the same time as August, arrived at a formula 

 identical in form with August's, but differing in the value assigned to 



g 

 the factor — --. Clerk Maxwell afterwards constructed a more elaborate 

 L a 



formula, in which the effects of radiation and diffusion were taken 

 into account, but which, by neglecting small quantities of the second 

 order, reduces into a form similar to that here given. 



To enable us to deduce with precision the hygrometic condition of 

 the air from the readings of the dry and wet bulb thermometers, it is 

 therefore only necessary to determine exactly the value of the constant A 

 of the formula / = / — A (t — f) h. With the best values of s, d, and L 

 known in 1845, the value of Ay when the thermometers are centigrade, 



becomes r— — -p, a formula giving for ordinary temperatures results 



differing very little from those computed by Apjohn's formula, in 

 which a constant value of the latent heat of evaporation is assumed. 

 To verify this formula (which, with the true values of the three quan- 

 tities entering into the factor A, as afterwards determined, should have 

 0'38 in the numerator instead of 0*429), Regnault instituted a long and 

 very careful series of comparisons between the indications of a psychro- 

 meter placed outside his laboratory window and the results obtained by 

 means of a chemical hygrometer through which air from the space be- 

 tween the two thermometers was drawn. The degrees of humidity 

 calculated from the psychrometric observations by means of the formula 

 were found to be in almost every case too high ; but, when the degree 

 of humidity was above 40 per cent,, the psychrometer was found to give 

 results in close accordance with the truth when the numerator of A was 



