on the Formula for measuring Heights by the Barometer. ^57 
the tension of vapour with the degrees of a hygrometer, has been 
accurately investigated only with respect to one of these in- 
struments, namely, the hair hygrometer of Saussure ; and that, 
too, at a particular temperature, so that the deductions drawn 
from it can scarcely be extended to cases in which the tempera- 
ture is different. The method of Dalton, too, though suscepti- 
ble of great precision, when the necessary experiments are per- 
formed with care, is not practicable in all circumstances ; nor are 
the objections to which it is liable greatly obviated by the hy- 
grometer of Daniel, which is intended to supersede it. The 
law, however, by which the evaporation of water is regulated, in 
a medium either absolutely dry, or partially charged with hu- 
midity, will enable us to determine the elastic force of vapour 
with greater correctness and simplicity, than it can be deduced 
from the indications of a hygrometer ; while it will not require 
so much calculation for computing the ultimate result, as the 
method of Dalton renders necessary. When I make this re- 
mark, I by no means wish to be understood as having the slight- 
est intention of undervaluing, in any respect, the labours of 
that eminent and philosophic chemist. On the contrary, the me- 
thod which I am about to propose, for finding the elasticity of 
atmospheric vapour, is founded upon one of his most valuable 
discoveries respecting the laws of evaporation ; and I should not 
do him justice, did I not embrace this opportuntity of remark- 
ing, that it affords, in my opinion, the surest basis for an accu- 
rate investigation of the hygrometrical constitution of the atmo- 
sphere, that has yet been proposed. The result of an investiga- 
tion, such as I allude to, was given some time ago in the Edinburgh 
Encyclopaedia, Article Hygrometry, p. 583 ; and I shall now 
briefly develope the physical and mathematical principles on 
which it rests. 
Since water, during its transition to the state of vapour, ab- 
sorbs as much caloric as would raise its temperature, had it con- 
tinued in the liquid condition, about 900® of Fahrenheit’s scale ; 
it follows, that the evaporation of a 900th part of a portion of 
that fluid would depress the temperature of the remaining mass 
* The investigation to which I allude, is contained in Biot’s Traite de Phy- 
sique, tome ii. p, 197., and possesses all the precision, perspicuity and elegance, 
which characterise the physical researches of that distinguished philosopher. 
