fr(yin the Indications of the Wet-hulh Hygrometer. 18S 



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 t\ 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 

 t\ if saturated. This being accomplished, \^f' be the tension 



1 \ f "= f'i the ten- 

 sion of vapour at /", the dew-point. This expression is strictly 

 correct, for it is obviously deduced from the fact of the ten- 

 sion of vapour at a given temperature and under a given vo- 

 lume being proportional to its quantity or specific gravity. 



1 \f may be found in the 



following manner. Unity representing the specific heat of 

 water, '267 (Delaroche 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 to vaporize 



a quantity of water represented by -rj^ = j, g-^ grains ; or, 



multiplying by the denominator, 4195 grains of water in 

 cooling through d degrees give out the exact quantity of heat 

 which constitutes the caloric of elasticity of d grains of va- 

 pour. But the volume of this weight of air at 60°, and under 

 a pressure of 30, is 13754? cubic inches; and at the tempera- 



448 + ^' 30 



ture t\ and pressure^, 13754 x X — = 27 X 448 -f^' 



508 p 



X — cubic inches. Hence 

 P 



30 



P 



xy = the quantity of moisture which the air contains 



^27x448i^'x— )x (l0-583x^-|^, X-305*)=87x 



♦ 10-583 x ./g^ X '305 = weight of a cubic inch of vapour whose 

 tension is /' and temperature t'. 



