98 REPORT -—1840. 
space is lowered accordingly. Further, it will be more lowered 
if the air be rare, or the barometer low, because the air yields 
less heat. Ail these circumstances may be taken into account, 
and the elasticity of vapour existing in the air may yet be ex- 
pressed by the following simple formula* :— 
ae AL cabo 2 es 
e =e —m(t—?t) B 
where ¢ and ¢ are the readings of a dry and a wetted ther- 
mometer ; 
é the maximum elasticity of vapour corresponding to 7 ; 
e! ” 2” » to the 
dew-point ; 
b the observed height of the barometer ; 
B a standard barometric pressure (as 30 inches). 
The concluding factor is usually small, and except at great 
heights may be neglected. 
176. Whene", the elasticity corresponding to the dew-point, is 
found, the quantity of vapour existing in the atmosphere is known. 
177. The chief value of a dew-point instrument is to enable 
us to determine directly the value of m, which the direct experi- 
ments on the specific heat of air leave under some uncertainty. 
178. Conversely the specific heat of air may be determined 
from hygrometric observations. This has been done by Dr. 
Apjohnt. 
179. The value of m manifestly depends upon the unit of elas- 
ticity, and upon the unit of temperature. 
180. Dr. Apjohn, of Dublin, whose attention was directed to 
the subject by the suggestion of the Committee of the Associa- 
tion, has, in a series of interesting papers, tested the value of m 
in a variety of circumstances by his own experiments}, and by 
those of others§. Assuming De la Roche and Berard’s value 
of the specific heat of air, he finds m, for English inches of 
mercury, and for Fahrenheit’s degree, to be gy = °01149: a 
posteriori he has determined it— 
(1.) from experiments on the dew-point . . . ‘01151 
(2.) es on refrigeration in dry air. °01150)| 
* This formula, employed by Auguste and Bohnenberger, coincides essen- 
tially with that of Ivory (Phil. Mag. Ix. 80.), who first gave a proper theory of 
the moistened bulb hygrometer. His value of m is not far from the truth, 
being ,1, for cent. degrees, or >, for Fahrenheit. 
+ Irish Trans., Phil. Mag., and Brit. Assoc., Sixth Report. See also Suer- 
man’s Thesis. 
+Phil. Mag., 8rd Series, vii. 266. 470. § Ibid., vi. 182. 
|| Prof. Lloyd has ingeniously made this set of experiments the means of 
testing the accuracy of the tables of the force of vapour, and he prefers Dalton 
and Ure’s calculated by Anderson.— Proceedings, Royal Irish Academy, 1840. 
