79 
On the Hygrometric Scale of the wet-bulb Thermometer. 
Wet-bulb Thermometer in a current of dry Air. 
Barometer. 
Thermometer 
in receiver. 
Wet-bulb. 
Therm. 
AqueousTension 
by Hygrometer. 
Depression. 
29.43 
72°5 
47°2 
.015 
25°3 
.52 
75.0 
48.2 
.015 
26.8 
.30 
78.5 
52.1 
.015 
26.4 
.26 
82.6 
54.8 
.015 
27.8 
.25 
83.5 
54.5 
.013 
29.0 
.30 
84.7 
55.0 
.015 
29.7 
.30 
85.0 
55 0 
.015 
38:0 
.20 
85.0 
54.8 
.012 
30.2 
.15 
90 2 
56.8 
.015 
33.4 
.15 
90.3 
56.7 
.015 
33.6 
Wet-bulb Thermometer suspended over concentrated sulphuric Acid in a 
close Vessel. 
Barometer. 
Thermometer 
in receiver. 
Wet- bulb. 
Therm. 
AqueousTension 
by Hydrometer. 
Depression. 
29.10 
90.0 
59.7 
303 
109 0 
66.7 
42 5 
1160 
70.0 
460 
125 0 
73 9 
61.1 
134. 
76 0 
58.0 
138. 
770 
610 
To compensate the want of perfect dryness in the air, the depressions in the 
last column must he increased from 0.5 to 0 7 of a degree Another correction is 
necessary for the height of the barometer. The amount may be estimated from a 
table given in Mr. Daniel’s Experiments on Evaporation, (Journ. Arts, XVTI.) 
wherein that gentleman makes the amount of evaporation as nearly as possible in 
the inverse proportion of the pressure, while the temperature of the water on the 
wetted bulb decreases arithmetically for geometrical decrements of pressure. 
Pressure 
Inches. 
Ratio. 
Evapora- 
tion. 
Grains. 
Ratio 
nearly 
Depres- 
sion of 
Tempe- 
perature. 
Ratio. 
Depression 
in Brews ter ’8 
Encyclopaedia, 
Art. Hy- 
grometry. 
6 
30.4 
i 
1 24 
i 
9 
0 
5 
0 
15.2 
i 
2.97 
2 
12 
a 
9 
a 
7.6 
i 
5.68 
4 
lo 
2a 
13 
2 a 
3.8 
9.12 
8 
18 
3 a 
18 
3 a 
1.9 
15.92 
16 
21 
4 a 
.9 
29.33 
32 
24.5 
ba 
.5 
5074 
64 
26 
6a 
Applying the rule, which seems to be pretty well established by these experi. 
ments, we shall find the correction dae to a change of pressure from 30 to 29 
inches, is at 90° only one-third of a degree; and therefore, fer trifling variations 
of the barometer, this correction may be neglected. It appears from the ex- 
periments above, that the depressions of the wet-bulb thermometer in a per- 
fectly dry air form a geometric series, increasing with the temperature. They 
may be neat ly represented by the formula 
IiOg. D =Log. T % 1.275—0.950, but it is better to adhere to the experimental 
numbers, where practical uses are alone contemplated, and I therefore prefer form, 
ing, by means of a diagram, a system of numbers which shall approximate as nearly 
as possible to them : this is easily done by varying the difference gradually, to 
render the curve uniform, as is shown in the fourth and fifth columns. 
