On the Hygrometric Scale of the wet-bulb Thermometer. 
81 
The numbers in the last column are seen at once to be of geometrical incre. 
meat ; that is, while the quantity of vapour in the air diminishes, the depression 
increases as some power of this diminution. It is not worth while to seek an 
exact formula to represent the ratio*, (the exponent of which is, however, 
about 1.6) ; for by a diagram, or by trial and correction of the second differences, a 
series may be interpolated quite accurate enough to serve all practical purposes : and 
the depression being thus found, say for every 10 per cent, of aqueous tension, 
(whose maximum is called 100,) at the temperature of 90, the table may be extend - 
edto other temperatures by the proportions already discovered of the maxima de- 
oressions corresponding to them. 
Depressions of the wet. bulb Thermometer in Degrees. 
Tension of 
aqueous 
Vapour. 
Temp. 
90 
80 
70 
60 
50 
40 
30 
20 
10 
0 
30° 
0°.6 
1°.3 
2°.0 
2°-7 
3°.5 
4°.4 
5°.3 
6°.4 
7° 6 
9°.0 
40 
0.9 
1.7 
2.7 
3.8 
4.9 
6.1 
74 
8.9 
10 6 
12.6 
50 
1.1 
2.3 
3.5 
4.9 
63 
79 
0.6 
11.5 
13-7 
16.3 
60 
1.4 
2.8 
4.4 
6.0 
7-8 
9.7 
11.8 
14.2 
169 
20.1 
70 
17 
3.3 
5.3 
73 
9.4 
117 
14.3 
17-2 
20.4 
24.2 
80 
20 
40 
6.2 
85 
11.0 
13 8 
16.8 
20.2 
25 1 
28.5 
90 
2 3 
4.7 
7.2 
99 
12 8 
16.1 
19 6 
23.5 
28 0 
33.1 
100 
2-7 
5.4 
8 3 
11.4 
14.7 
18.4 
22.4 
270 
32 1 
38.0 
110 
30 
6.1 
9.4 
13.0 
16.8 
21.0 
25.6 
30.7 
36.6 
43 3 
120 
3.4 
7-0 
10.7 
14.7 
19.1 
23.8 
290 
34 9 
41.4 
49 1 
From the foregoing table, an inverted one may be formed, showing the 
tensions corresponding to given depressions, but it will answer sufficiently well 
as i t is for the same purpose, with a little more trouble only in making the interpo- 
lations. 1 shall conclude with a few examples of its application to the observations 
of different individuals the data of the first five are taken from the article 
Hyobomktuy in Brewster's Encyclopedia; the succeeding four, are from observa- 
tions male by an officer on the river ; the remainder are from my own register (the 
tensions of the first five are derived from the hair hygrometer.) 
* The author of the articleHronoM-ETitv in the Edinburgh Encyclopedia, gives 
a formula for the depression of the wet.bulb, dependant upon the tension of vapour 
at the given temperature, of which he argues that the evaporation, and consequent- 
ly the cold produced thereby, must be a function. Assuming F the maximum 
tension, / the actual tension by the dew point, B and 5 the standard and the actual 
pressures, and p and r quantities determined by experiment, he makes 
D = T (p -^ )(F - /) 
This formula suits the temperature at which it seems to have been adopted 
(about 65° i but unfortunately no other ; for if we calculate the maxima depressions 
for the temperatures of 30, 60, 90, and 120, rejecting from the formula B 5 and 
and calling p = 36 and r = 10 as determined by the writer; we shall find 1) 
respectively equal to 7-0°, 28°, 45°, and 103°, instead of 9, 20, 33, and 49 as proved 
^The table constructed by myself may be nearly represented by a formula de- 
pending on the temperature and pressure, the latter of which may generally be 
neglected. 
d=(.001 t »•«)-*- (.112 /‘' 275 )X */—■ 
or where the depre ssion i s given, the tensi on t will be 
t= , ' * 1 OOOfl / F 
V .112 /‘■s’ , x'V p 
in this expression i is the complement of the tension, (making the maximum tensi- 
on 100,) f is the temperature, and d the depression of the wet bulb observed. The 
formula becomes less accurate as the depression approaches zero. 
