British Association for the Advancement of Science. 313 



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 t\ 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' = d 

 degrees, then the relation in question is expressed by the equation 



f" ~f ( ) in which f" is the tension of vapour at the dew- 

 point, andy*' 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 f 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 



d n 

 is converted into the following : fv =f ( — — x 5jr« 



To this solution of the dew-point problem it may be objected that 

 the coefficient which is set down as -»y cannot be constant, in as 

 much as its value depends upon the latent heat of aqueous vapour 



* The substance of Dr. Apjohn's first paper on this subject appeared in 

 Lond. and Edinb. Phil. Mag., vol. vi. p. 182 ; and that of his second paper 

 will be found in our present number, p. 266. — Edit. 



t t is the temperature of the air, and t' that shown by the wetted instru- 

 ment. 



Third Series. Vol. 7. No. 40. Oct. 1835. 2 S 



