On Hygrometry. 
193 
From this series the following is easily found by interpolation and corrected for 
barometer 29,27. 
Temperature of air. 
Depression of moist 
Bulb. 
Depression from 
Table p. 80. 
Difference. 
140 
61,3 
62,6 
1,3 
130 
55,1 
55,5 
0,4 
120 
49,2 
49,1 
0,1 
110 
43,5 
43,3 
0,2 
100 
38,2 
38,0 
0,2 
90 
33,1 
33,0 
0,1 
80 
28,4 
28,5 
0,1 
70 
24,0 
24,2 
0,2 
60 
20,0 
20,1 
0,1 
Excepting the first, which, as I said before, appears to include some anomaly, the 
differences are here within the errors of observation. 
The above may be sufficient to show the value and the uses of the formula. I am 
now constructing tables from which the hygrometrical state of the air shall be rea- 
dily found, the temperature of the air and of a moist evaporating surface being given. 
The table given by your correspondent page 81 founded on experiment is almost 
sufficient ; nevertheless it will be interesting to compare the results of the formula 
with those of observation. I may add, for the use of those who observe with 
Leslie’s form of the instrument, that we must multiply the constant 6.056 
by or increase the logarithm 0.7822 by 0.9122= 1.6944 the natural 
number of which is 49.5. Adapted to Leslie’s hygrometer then, the formula (D) 
will become 
49,5 H " !,, B „ 
L20 f 
where H is the indication of Leslie’s hygrometer; F is, as throughout this paper and 
the preceding, the force of vapour due to the temperature of the evaporating surface ; 
and /, that of the dew point. As this last is the quantity sought, it may be more con- 
veniently given _ ^ 49,5 H—B (G) 
f ^ L. 30. 
L always corresponds to the temperature of the moist surface. This temperature 
in the case of observing Leslie is = temperature of air— H X 9 -r 50 ; 50° of Leslie’s 
scale being equal to 9° of Fahrenheit. 
Postscript. 
The value of the co-efficient 1.233 I thought it preferable to take from Dulong 
and Petit, as their experiments appear to have been conducted with such care. It 
would however be easy to deduce them from the experiments given p. 80, as follows. 
The equation (D) being divided by the constants, we shall have in the case of 
dry air when/ - = O, putting « = 1.233 
1J ”_ • T' ’ F'or D“ :D'“ : : F L : F" L' that is 
L • L' “ 
, logarithms, n (Log. D'— Log. D) = Log. F'' + Log. L'-Log. F'+Log. L 
D'° 
' D 
F''L' 
: fTT 
Log. F’ -f- Log. L'— Log. F— Log. L __ ^ 
Log. D' — Log. D 
In this way I calculated the value of n for each of the observations page 80 as compar- 
ed with the first, and found the following values 1.204, 1.230, 1.267, 1.266, 1.267, 
1.252,1.222, 1.181, mean 1.236, or, rejecting the last, 1. 244. The first is only 
003, the second 011 more than Dulong and Petit’s index; and this excess may be 
