of the Moist-bulb Hygrometer. 277 
Having disposed of these preliminary observations, I shall now proceed to explain 
the principle of my method of investigation ; but before doing so, I wish to observe 
that I would have submitted my formula to the Committee of the British Association 
in Edinburgh, charged with the subject of Meteorology, but that, having fallen upon 
it during the vacation, while sojourning in the south of Ireland, I was not able, prior 
to the meeting, to institute test observations myself, nor had I access to books, so as 
to compare it with the recorded experiments of others, on the temperature of the 
moist-bulb Hygrometer and the corresponding Dew-point. 
When in the moist-bulb Hygrometer the stationary temperature is attained, the 
caloric which vaporizes the water is necessarily exactly equal to that which the air 
imparts in descending from the temperature of the atmosphere to that of the moistened 
bulb; and the air which has undergone this reduction becomes saturated with 
moisture. Now from these facts, and the known specific heat of air, we can calculate 
the weight of water m which would be converted into vapour by the heat which a 
given weight of air would evolve in cooling from ¢ the temperature of the atmosphere 
to ¢ that of the moistened bulb; and we can also calculate the total quantity of 
moisture m which the same weight of air would contain at ¢ if saturated. This being 
accomplished, if f’ be the tension of vapour at the temperature ¢ (1 —=) f= Fe 
the tension of aqueous vapour at ¢” the Dew-point. Hence, by looking in Dalton’s 
table for f”, the Dew-point is found in the opposite column. 
The value of the’ expression (1 _= ) f’ may be found in the following manner :— 
1 being the specific heat of water, .267 (De la Roche and Berard) is that of air. Also 
967° being the caloric of elasticity of steam at 212°, 212—50+967=1129° will be 
its caloric of elasticity at 50°, assuming, as is generally done, that the sum of the 
sensible and latent heats of vapour is the same at every temperature. One grain of 
air, therefore, in cooling through any number of degrees d, will raise the temperature 
of .267 grains of water through the same number, and will consequently be adequate 
: 267d d : ey os 
to vaporize a quantity of water represented by Sy55= 4195 grains ; or, multiplying by 
the denominator, 4195 grains of air, in cooling through d degrees, give out the 
exact quantity of heat which constitutes the caloric of elasticity of d grains of 
vapour. But the volume of this weight of air, at 60°, and under a pressure of 
30, is 13754 cubic inches; and, at the temperature ¢ and pressure p, 13754 
Baste 30. 4484¢ ... 44847 Ee 
508° 5 =810 cubic inches. Hence Cin, x (10,583 x MASTe 
* This expression is obviously deduced from the fact of the tension, of vapour at a given temperature 
and under a given volume, being proportional to the quantity or specific gravity. 
