TRANSACTIONS OF THE SECTIONS. 
27 
Results of a Third Series of Experiments on the Quantities of Rain 
received at different Heights in the Atmosphere. By W. Gray, 
Jun. and Prof. Phillips. 
[This Paper will be printed in the next volume of Transactions.] 
Dr. Apjohn explained the substance of two papers recently read 
by him before the Royal Irish Academy, and which have within a 
few days appeared in the last part of their Transactions. In the first 
of these papers a formula is investigated for pointing out what has 
long been considered a desideratum in meteorology, namely, the ex¬ 
act relation between the indications of the wet-bulb thermometer 
and the corresponding dew-points ; while, in the second, a number of 
experiments are detailed, instituted for the purpose of testing its ac¬ 
curacy, and which seemed to prove that the formula represented 
observations with an extreme precision. The following is an out¬ 
line of his communication, which was made orally to this Section. 
When the wet thermometer attains its stationary temperature, 
the caloric which it loses and acquires in a given time are obviously 
equal. The latter is that imparted by the surrounding air to the 
instrument in cooling through t — l' f degrees, and the former that 
which constitutes the caloric of elasticity of the vapour formed. Now 
if m be the amount of moisture which a given weight of air is ca¬ 
pable, when saturated, of containing at the temperature and m' the 
quantity of vapour which would be formed at the same temperature 
by the caloric evolved from the air in cooling through t — t' =z d 
degrees, then the relation in question is expressed by the equation 
f" —f ^ in which f n is the tension of vapour at the dew¬ 
point, and f its tension at the temperature of the wet-bulb thermo¬ 
meter. This expression is rigorously exact, for in arriving at it we 
merely assume what must at once be conceded, namely, that the air 
which is cooled by contact with the moist bulb becomes saturated 
with moisture at the temperature t', and that the tension of vapour 
at a given volume and a given temperature is directly proportional 
to its quantity or specific gravity. But the value of m is easily as¬ 
signed by aid of the theory of mixed gases and vapours, and that of 
m' also admits of being readily deduced from the known values of 
the specific heat of air and the caloric of elasticity of vapour. When 
this is done, and the proper substitutions made, the above expression 
is converted into the following: f" —f — ^ x 
To this solution of the dew-point problem it may be objected that 
the coefficient which is set dowm as -sV cannot be constant, in as 
much as its value depends upon the latent heat of aqueous vapour 
and the specific heat of the medium which encompasses the wet 
thermometer, both of which vary, the former with the temperature, 
and the latter with the pressure and the amount of vapour present 
in the air. Such objection is theoretically just, and the necessary 
corrections have therefore been investigated by the author in his 
