THE ATMOSPHERE. 277 



making two or three rapid strokes of the pump, which is due to 

 this cause. It is observed in operation in the formation of 

 the clouds and mists which settle on the summits of mountains. 

 The wind passing over the surface of a level country is impeded 

 by a mountain ; rising in the atmosphere the stream overcomes 

 the obstacle, and produces a cloud as it passes over the moun- 

 tain, which appears stationary on its summit. 2. The mixing 

 of opposite currents of hot and cold air, both saturated with 

 humidity, may occasion rain, from the circumstance, first con- 

 jectured by Dr. Hutton, that the currents of air on mixing and 

 attaining a mean temperature, are incapable of sustaining the 

 mean quantity of vapour. Thus, supposing equal volumes of 

 air at 60 and 40, both saturated with vapour, to be mixed ; 

 the tension of vapour at the former temperature being the 

 0.524th of an inch of mercury, and at the latter the 0.263rd of 

 an inch, the mean tension is the 0.393rd of an inch. But the 

 tension of vapour at 50, the intermediate temperature is only 

 the 0.3 / 5th of an inch ; and consequently the excess of the 

 former tension, or vapour of the 0.018th of an inch of tension, 

 must condense as rain. But this is an inconsiderable cause of 

 rain compared with the next. 3. Contact of air in motion with 

 the cold surface of the earth, appears to be the most usual cause 

 of its refrigeration, and of the precipitation of rain from it. 

 The mean temperature of January in this country is about 34, 

 but with a south west wind the thermometer may be observed 

 gradually to rise in the course of 48 hours to 54. Now sup- 

 posing this wind to be saturated with vapour at 54 and to be 

 cooled to 34, as it is on its first arrival, the moisture which it 

 will deposit is very considerable, as will appear by the following 

 calculation. 



Tension of vapour at 54 . . 0.429 inch. 

 at 34 . . 0.214 



Condensed . . 0.215 



Wh en clouds form at temperatures below 32, the aqueous 

 vapour is converted into an infinity of little needle-like crystals, 

 which often diverge from each other at angles of 60 and 120, 

 as do also the thin crystals in freezing water. Snow differs very 

 much in the arrangement of these spicuke, but the flakes are all 



