276 Professor Apsoun on the Theory 
porated ; and it is obvious that the amount of this depression will bear some direct 
ratio to the degree of dryness of the air at the time of the experiment. Further than 
this, however, the wet-bulb Hygrometer does not go. It affords us, for example, no 
information as to the exact quantity of moisture present in the surrounding atmo- 
sphere at the time of observation, because it does not indicate the position of the Dew- 
point. That there existed, however, between this latter and its indications, some 
necessary connexion, so that the one might be inferred by calculation from the other, 
must have been very early suspected; but Sir John Leslie, after having converted 
his differential thermometer into an hygrometer, was the first who attempted to point 
it out. In this attempt, I believe I may venture to say, he was but partially success- 
ful. I have never seen any very explicit statement of the principle on which he pro- 
ceeded ; but the table which he published in 1820, in his “ Description of Meteoro- 
logical Instruments,” is undoubtedly erroneous. 
In a very able article on Hygrometry in Brewster’s Encyclopedia, a formula for 
the solution of this question is elaborately investigated ; but as it is in greater part 
tentative, assumes, contrary to the fact, that the amount of moisture which air is 
capable of taking up is influenced by the pressure; and, as lastly, it does not very 
well accord with observations—at least some which I have made—it has been 
adopted, I believe, but by few Meteorologists. A satisfactory solution of this problem 
was still viewed as a desideratum, and of this no better proof can be given than that 
the “theory of the moist-bulb Hygrometer” is found among the questions submitted 
by the first meeting of the British Association, held at York, to the renewed consider- 
ation of philosophers. 
About four months since, on turning the matter in my mind, it occurred to me 
that the relation between the Dew-point and the temperature of a thermometer with 
moistened bulb might be made matter of calculation, and deduced from the theory of 
mixed gases and vapours—a theory which the labours of Dalton and G. Lussac have 
rendered as complete as any other in Physics. It was not, however, until the latter 
end of August that I was enabled to return to the subject, when I succeeded in 
arriving, by a direct rout, at a formula derived exclusively from experimental data, 
and which represents with unexpected accuracy, the best observations with which I 
have been able to compare it. To this formula (to explain, at length, the object of 
this hurried communication,) I am anxious to draw, without further delay, the atten- 
tion of men of science, as there is a brief notice of one for accomplishing the same 
object in the article of Professor Jamieson’s Journal already alluded to. Of this 
latter, which is said to belong to Mr. Ivory, I beg to say I was altogether ignorant ; 
nor haye I yet been able to refer to the number of the Philosophical Magazine in 
which it is stated to have been originally published. 
