80 On the Hygrometric Scale of the-wet bulb Thermometer. 
Table of maximum Depressions. 
Temperature. 
Calculated. 
Observed. 
Nearest approx. 
Diff. 
30° 
8.6 
9.0 
9.0 
3.6 
40 
12.4 
12.7 
12.6 
3.7 
50 
164 
16.1 
16.3 
3.8 
60 
20 7 
19.9 
20.1 
4.0 
7» 
25.2 
24.0 
24.2 
4.3 
80 
29 9 
28.4 
28.5 
4.6 
90 
34 7 
34.0 
33.0 
4.9 
100 
39 7 
— 
38 0 
5.3 
110 
44.9 
43.5 
43.3 
5.8 
1-20 
50.2 
49.2 
49.1 
6.4 
130 
55.6 
55.4 
55.5 
7-1 
140 
61.0 
Having then established the two extreme points of the required hygrometric 
scale, we may proceed to fill up the interval by means of series of comparative ob- 
servations with the dew point hygrometer of which notice was made in the com- 
mencement. Unfortunately, the temperatures at which these comparisons were 
made, were almost entirely between 80° and 95° Farqnheit, hut it is reasonable to 
suppose that the same law which will accommodate this range of observations 
would hold good for other temperatures; and it will, therefore, l>e sufficient to settle 
with accuracy a single line of hygrometric tensions ; and that for the temperature 
of 90 degrees, is the one to which our register can be most conveniently adapted. 
I shall not attempt to copy at length the whole series, which were made between 
March and September, embracing, therefore, the driest and dampest state of the air. 
Each observation was inserted in a general diagram, comprehending the tempera- 
ture, the depression, and the aqueous tension as given by the dew point hygrometer. 
This diagram presented to the eye several group s of the most prevalent states 
of the air, with regard to moisture; and from each of these could be derived an 
average, as a fixed point in the required table of values. 
The figures in these groups varied among themselves four or five per cent, and 
frequently more, which proves the amount of uncertainty due to determinations 
by the wet-bulb : but it must be remembered, that this variation also includes the 
errors of the dew point process, which every observer knows to be liable to a consi- 
derable range of uncertainty : in fact, it is very probable, tli at the differential or wet- 
bulb thermometer kept constantly in action, may give a fairer average state of the 
air, than the momentary and somewhat troublesome experiment of finding the 
dew point, even by the rudest table which we may construct. 
I have thought it right to confine m yself in the following table to dew point com- 
parisons : I had collected others with the hair hygrometer for the period of a year 
and a half, and they in a great measure agree with these ; but the deductions from 
premises in a measure doubtful will always be looked upon with suspicion, whereas 
of the infallible principle at least of the dew point experiment there can be no 
doubt whatever. 
Table of Aqueous Tensions observed. 
Temp. 

Number Obs. 
Wet. bulb. 
Depression. 
Aqueous 
Tension. 
Depression reduced 
to temp. 90° 
7 
12 
12 
6 
6 
13 
8 
by former table. 
85° 
87.5 
90. 
94. 
92.5 
87.0 
92. 
90. 
3°5 
5-7 
9.5 
13.0 
170 
20. 
24. 
33.1 
83.1 
76.1 
65.4 
52.3 
41.0 
26.0 
17-9 
0 
3°7 
5.9 
9 53 
12 3 
16.8 
20. 
23.3 
33.1 
