On the Hygrometric Scale of the wet-bulb Thermometer. 
77 
Original Communication#. 
I.— On the Hygrometric Scale of the wet-bulb Thermometer. 
On a former occasion*, I ventured to give an approximate table of the values of 
the depressions of the wet-bulb thermometer ; but as I had not then a sufficient 
series of comparisons with the dew point hygrometer, to establish the soundness of 
my views, I now return to the subject prepared to supply this defect, and more 
confident, at the same time, that no instrument can so well be applied to the meteo- 
rological branch of hygrometry. _ , 
There are two methods of expressing the hygrometric state of the air ; either by 
the absolute quantity of aqueous vapour existing in a given space ; or by the pro- 
portional tension of the vapour compared to the maximum moisture, capable of 
being sustained by the air at any temperature ; the latter is in every respect the 
most convenient expression, since the quantity of moisture varies for every tempe- 
rature, and it may be readily obtained from the usual tables, when the tension and 
the heat are registered. 
As in all other hygrometers, then, two points, namely, those of extreme mois- 
ture and extreme drought, require to be ascertained, the space between which may 
be divided into one hundred commensurable parts, to be called the scale of tensions* 
The first of these is self-evident ; for, when the air is fully saturated with mois- 
ture, evaporation, and consequently the cold produced by it, is = 0. This point is, 
however, generally termed 100 on the scale, whence it may be premised, that in find- 
ing a formula for the tensions depending on the depression of temperature of an 
evaporating surface, the complement of the tensions will be the most convenient 
index. 
To find the other fixed point of the scale, or the diminution of temperature 
which would attend evaporation in an atmosphere perfectly dry, i. e. the maximum 
depression, is by no means so easy a problem, since the air is never altogether devoid 
of moisture in the driest weather, neither is it readily deprived thereof by artifi- 
cial means. , , . 
M. Gay-Lussac at one time turned his attention to the solution of the wet-bulb 
problem, and it is to be regretted, that this able philosopher should have quitted the 
field, (no doubt 'for others of vaster interest and importance,) before his investiga- 
tions were completed. He has, however, left us, in the Annales de Chimie, a valuable 
table of depressions obtained by passing a current of perfectly dry air over a ther- 
mometer with a moistened bulb. They are as follows, converted into Faren ei 
degrees. 
Temperature. 
Depression. 
32.° 
9-°98 
33.8 
10.96 
40. 
12.7 
50. 
16.1 
55. 
18.0 
60. 
19.9 
65. 
21.8. 
70. 
24.0 
75. 
25.7 
As is usual with experiments made in Europe, this scale is unfortunately not 
extensive enough to be adapted to our climate , at any rate, it appeared desirable 
to confirm, by actual trial, the rate of progression which might be deduced from it 
for higher temperatures. This object was pursued in the following manner. 
* Or. Quart. Mag. 
