CLIMATE. 75 



deposit, that is, the heaviness of the shower, being deter- 

 mined by the disproportion between the relative density 

 and temperature of the two strata, and their being each 

 nearly saturated, or not, with miosture. An example will 

 render this more distinct. Let us suppose that a current 

 of air at the temperature of 25° (disregarding density for 

 the sake of brevity,) meets another current of the tempera- 

 ture of 15°, and that both are fully charged with moisture. 

 When mixed, their mean temperature will be 20°. Now, it 

 has been ascertained by experiment that air, (i. e. a cubic 

 mass of it 40 inches each way) at 25° can hold in solution 

 317'5 grains of moisture, and at 15° 200 grains, the mean 

 of which is 258' 75 ; but at the mean temperature of the two 

 currents, viz. at 20°, air can only contain 252 grains ; there- 

 fore 8'75 grains must be precipitated either in the form of 

 clouds, fog, or rain. Saturation of the air with moisture, 

 so as to produce deposition on any further diminution of 

 heat, may also be produced by a simple reduction of the 

 temperature of the column below the point at which it can 

 hold its moisture in solution. Thus, a column of air at the 

 temperature of 15° with 180 grains of water in solution, if 

 reduced by any cause to the temperature of 12'8, does not 

 deposit any moisture, being still capable of dissolving 180 

 grains ; but if a further reduction of 2° take place, a deposi- 

 tion of 35'3 grains ensues. 



Upon these simple facts are founded a number of curious 

 experiments and observations, such as the quantity of mois- 

 ture contained in the air at the time of observation, its dry- 

 ness or the quantity of moisture required to saturate it, and 

 thirdly, the reduction of temperature required to produce 

 saturation, and consequent deposition, or the dew-point, as 

 it is called. 



L 2 



