954 
Proceedings of the Royal Society 
Again, at a wet bulb temperature of 32° F., we find a sudden change- 
in direction, the lines becoming much steeper. This change is 
apparently discontinuous, but as the wet bulb temperature continues 
to fall the slow increase in steepness continues as before. 
These results lead to the linear equation 
/' -f" = (t-t')K, 
where f is pressure of vapour at temperature of wet bulb, f" is 
pressure of vapour at temperature of dew-point, t temperature of 
air, f temperature of wet bulb, and K the factor before referred to^ 
whose values are to be determined. 
It is certainly premature to attempt to give any definite values of 
R, but by drawing lines in the diagram at those temperatures of wet 
bulb at which any considerable number of observations appear, as 
nearly as possible in the mean direction indicated by the obser- 
vations, the following 
obtained : — 
roughly 
approximate values 
have 
Temperature 
of Wet Bulb. 
K 
1 Temperature 
of Wet Bulb. 
K 
45° F. 
i 
108 
33° F. 
i 
9 T 
o 
o 
1 
106 
[31° 
A] 
39° 
1 
T06 
[30° 
*1 
0 
00 
CO 
1 
10 6 
[29° 
tVI 
37° 
1 
10 T 
00 
cq 
A] 
36° 
1 
103 
[25° 
A-] 
0 
iO 
CO 
1 
10 1 
[20° 
A] 
34° 
1 
9 
[17° 
Tf] 
Until more continuous observations are made these values must not 
be taken as more than a first approximation. Below 32° they are 
probably too small. 
The next step was to compare the dew-points given by the hygro- 
meter with those calculated from the wet and dry bulb thermometer 
readings by the two methods in most common use. Glaisher’s 
“ Ilygrometrical Tables” (6th edition, 1876) and “Apjohn’s 
Formula.” For this purpose it was only necessary to fill in the 
diagram lines calculated by these two methods. 
Glaisher’s tables are constructed from factors obtained chiefly 
from the Greenwich Observatory observations with wet and dry- 
