78 On the Hygrometic Scale of the ivet-bulh Thermometer . 
A large gasometer containing 120 pints of air was well dried, and the reservoir 
thereof filled with pure cocoajmt oil instead of water, it having been previously 
ascertained under the pump that this oil did not give out the least .moisture, Above 
the oil and, consequently, within the air chamber, was suspended a large flat dish 
of concentrated sulphuric acid, which has the property of abstracting moisture more 
powerfully than any other substance. The gasometer was connected with the 
receiver of an air pump by tubes, so arranged that air could not pass into the 
receiver, without circulating through a large quantity of muriate of lime, which had 
been previously heated to redness. A thermometer and a hair hygrometer were 
placed within to note the heat and drought; and a stopcock at the top opened a 
communication with a small chamber containing the minute thermometer with a 
bulb covered with moistened cotton. By applying weights to the gasometer, a steady 
draught could be maintained through the apparatus, without the slightest variation 
of thp hygrometer, although the exterior air, from the season, was loaded with 
vapour. 
A certain velocity of the air was found to produce the greatest depression ; — from 
70 to 80 inches in the minute, which is hardly a perceptible breeze. The air was ne- 
ver completely dry, for muriate of lime does not appear capable of carrying the desic- 
cation lower than 1. 5, per cent, of aqueous tension or 9 degrees of the hair hygro. 
meter, at which point it continued from August to October without alteration. 
Upon opening the stopcock the wet thermometer began instantly to fall; and as 
the minutest circumstances in experiments conducted with tolerable care may always 
turn to some use, the rate of cooling was several times noted down during equal peri- 
ods oi time. It was thus ascertained that the curve which would nearly represent the 
rate of cooling, approached to the form of a quadrantal arc ; for, calling the time in 
which the maximum depression was attained, radius, the intermediate depressions 
were nearly as the sines or ordinates, to the abscisses of'the radius, or the interne 
diate times : in other words D ==' m \/ab where b = 2 radius a 
and a = the time, in parts of the radius. 
Time in minutes 
and seconds. 
0 
30 
1 
00 
1 
30 
2 
00 
2 
30 
3 
00 
3 
30 
4 
00 
4 
30 
5 
00 
epression wet-bulb 
degrees. 
Calculated depression 
in degrees. 
10° 
10°7 
16 
15.0 
19.2 
17-7 
21.3 
20.0 
22.6 
21-7 
23.6 
23.0 
24.3 
23.9 
24.5 
24.5 
24.9 
24.9 
25.0 
25.0 
de- 
that nf • . tne ^et-buJbat the beginning ot these experiments was i 
grees ™ exteri0r air 85 °> and the maximum depression a little more than 30 
exol-Wm! ™ marl ; wl in , the following table, that the depressions obtained by my 
exj enments are a degree m excess ot if. Gay-Lussac’s. This may either be owing 
Over S“rti gll f if ^ em P 1 °y™ , »t of a very minute thermometer. 
at .75% when the wet-hull, thermometer stt^Tat 6 4 5 fh'T'f m £5,' TL” “v 
agitation would lower it one degree, which Droves 1 found, that brisk 
to this cause of irretruJarirv The f„n„ P tllnt lp g ar d must he always paid 
thus conducted! % > * foUowm S 15 abstract of the experiments 
