4S^ Explanation of the Theory of the ^uly, 



slightest degree to the temperature of the air saturated with 

 moisture with which it may be in contact will be perceptibly 

 clouded with light dew. 



(39.) When the air is but partially saturated, we must conti- 

 nue to lower the temperature of the polished surface, and mark 

 the degree of the thermometer at which the deposition of the 

 dewjirst takes place. 



(40.) This temperature is termed the point of condensation, 

 dew-point, or constituent temperature of the vapour. 



(41.) The elasticity of the vapour contained in the air is 

 understood to be equal to that of vapour generated and existing 

 in its saturated state in a temperature equal to that of the 

 observed dew-point. The hygrometer of Mr. Daniell is an 

 elegant, but somewhat expensive instrument, to ascertain the 

 temperature of the point of condensation. 



(42.) When the air is not wholly saturated with aqueous 

 vapour, a minute portion of water of the temperature of the air 

 bemg isolated within it will evaporate with a sensible reduction 

 of its heat, determined by the degree of saturation, temperature, 

 &c. ; consequently a thermometer having its bulb covered with 

 humid bibulous paper, &c. will be observed at a temperature 

 inferior to that of the ambient air ; which difference, together 

 with the other data, easily procured, will enable us to calculate 

 the tension of the vapour contained in the air. 

 . (43.) Hygrometers constructed of organic substances elon- 

 gating from humidity indicate merely the degree of saturation ; 

 yet with this datum in addition to that of the observed tempera- 

 ture of the medium, we may ascertain the elastic force of the 

 vapour with sufficient exactness for barometrical calculations. 



Of the Density of Air containing aqueous Vapour, 



(44.) The densities oi equal volumes of different fluids are as 

 their weights ; and their weights, if elastic fluids having the 

 same temperature, will be as their elasticities, or the pressures 

 they sustain, multipHed by their respective specific gravities, 

 determined when their temperatures and elasticities were the 

 same. 



(45.) Recollecting that as much vapour is formed in the same 

 space, being a vacuum, or containing dry air, we may readily 

 find the density of air wholly or partially saturated with mois- 

 ture compared to that of dry air supporting the same pressure, 

 and existing in the same temperature, by adding together the 

 weights of the vapour and that of the dry air of the mixture. 

 T^e ., elasticity of the dry air being 15 inches, and that of the 

 vapour^ J t) inches, the observed pressure would be equal to the 

 s^/m of their elasticities, or to 25 inches. Calling 1 the weight 

 of a cubic loot of dry, air of this latter pressure (25 inches), an 



