242 
SECOND REPORT -1832. 
the glasses employed are tliin. This is the result at which Dr. 
Thomson of Glasgow has arrived*. 
The next point is the Table of the elasticities of vapour at dif¬ 
ferent temperatures. Mr. Dalton’s excellent Table, or that cal¬ 
culated by Dr. Young from the experiments of Dr.Ure, will be 
quite sufficient for the range of atmospherical temperatures. 
The new Table derived from the meritorious labours of the 
French savans, whose experiments have been carried up to a 
pressure of steam amounting to 24 atmospheres, will probably 
become the standard reference on this subject, at least in the 
case of high pressures f. The formula at which they have ar¬ 
rived, and which bears a striking analogy to that of Dr.Young, 
is the following: 
e=(l +0*7153 t)\ 
where e is the elasticity in atmospheres (reckoned at = 0 m *76), 
and t the temperature reckoned from 100° and computed in cen¬ 
tigrade degrees. 
From any such Table of elasticities, with Gay-Lussac’s result 
for the specific gravity of aqueous vapour, the weight in a cubic 
inch under any circumstances may easily be computed. It is 
hardly necessary, however, to repeat that the expression for the 
degree of humidity is not the actual weight of moisture in a 
given space, but the proportion which that bears to the weight 
which might exist without deposition under the circumstances 
of temperature and pressure. 
Great as are the advantages of simplicity of calculation, which 
the dew-point experiment affords, there is a less direct expe¬ 
riment which offers great facilities in performance and likewise 
the means of self-registration. I allude to the moistened bulb 
hygrometer, in which the coolness produced is a function of the 
dryness of the atmosphere, without bearing any relation to the 
force of wind or other circumstances which affect the rate of 
evaporation. Under the simplest form of two thermometers, 
one of which had its ball moistened, it was employed by Dr. 
James Hutton; and afterwards Professor Leslie adapted it to 
the principle of his differential thermometer ; a change perhaps 
not contributing to the simplicity of the instrument, which still 
requires a detached thermometer to determine the temperature 
of the air. Accordingly, the instrument in its first and simplest 
form (in which for years we have been in the habit of using it,) 
has recently been reproduced by M. Auguste, under the high- 
* Thomson On Heat, p. 256. 
f Annates de Chimie, Janvier 1830, tom. xliii. p. 74. 
