710 lECTUBE LVII. 



scale than eitlier of them. Mr. Leslie employs a very delicate thermometer, of 

 which the bulb is moistened, for measuring the dryness of the air, by the cold 

 produced during evaporation, when the thermometer is exposed to it; but this 

 mode of estimating the quantity of moisture appears to be liable to consider- 

 able uncertainty. (Plate XLI. Fig. 581.) 



In order that the scale of a hygrometer should be perfectly natural, it ought 

 to express, at all temperatures, the proportion of the quantity of moisture in 

 the air to that which is required for its saturation; thus, at 100 degrees, 

 it should imply that the slightest depression of temperature would produce a 

 deposition; at .50 degrees, that the air contains only half as much water as 

 would saturate it, or, supposing the thermometer at 52°, that a deposition 

 would be produced in it by a depression of 17°. And if we know the actual 

 temperature, and the temperature at which the deposition takes place, we 

 may find the height of the natural hygrometer, by the proportion of the 

 corresponding elasticities of steam. The mean height of the natural hygro- 

 meter in London is probably about 80°; that of Deluc's hygrometer,' with 

 proper corrections, being nearly 70°: so that a depression of 6° must usually 

 be sufficient to cause a deposition of moisture. 



The quantity of water actually contained in a cubic foot of air, saturated 

 with moisture, appears to be about 2 grains at the freezing point, 4 grains 

 at 48°, 6 at 60°, and 8 at 68°; and the density of the vapour, thus mixed 

 with air, is, according to Saussure's experiments, about three fourths as great as 

 that of the air itself; so that moist air is always a little lighter than dry air; 

 and the more so as the air is warmer, provided that it be saturated with mois- 

 ture by means of the presence of water. It follows from the properties of mois- 

 ture thus determined, that if any two portions of perfectly humid air, at differ- 

 ent temperatures, be mixed together, there must be a precipitation: thus, a cubic 

 foot of air at 32° being mixed with another at 60°, their common tempera- 

 ture must be 46°; if they are saturated with moisture, they must contain 8 

 grains of water when separate; but when mixed they will be too cold by 2° to 

 contain the same quantity; since air at 48° can only contain 4 grains for each 

 foot; and it has been supposed that such mixtures frequently occasion a pre- 

 cipitation in nature. Thus, it often happens that the breath of an animal, 

 which is in itself transparent, becomes visible when mixed with a cold atmo- 



