PNEUMATICS. 75 



portions of air of different temperatures, and satu- 

 rated, or nearly so, with moisture, cannot be inter- 

 mingled without its precipitation. This precipitation 

 in its turn is a fresh cause of wind, of the collision of 

 different airs, and of the renewed formation of clouds 

 and rain ; and the contraction that thus takes place 

 in the atmosphere diminishes its elasticity, lowers 

 the barometer, and again becomes a cause of wind. 



Evaporation. 



The evaporation of water is about equal to the fall 

 of rain. In ordinary soils 30 inches out of 38 escape 

 by evaporation. In the case of large rivers flowing 

 over extensive and level surfaces, the water carried 

 to the sea does not exceed one fifth of what falls in 

 rain, snow, and dew. 



Rainbows. 



Rainbows are only visible when the altitude of the 

 sun is between 45 and 56. In summer, therefore, 

 in this latitude, they are not seen about the middle of 

 the day. A dense halo round the moon portends 

 rain. 



Effect of Mountains on the Atmosphere. 



Mountains precipitate the moisture of the atmos- 

 phere, not so much by attracting it to their summits 

 as in consequence of their rocky and grassy sides, 

 when acted on by the sun, heating large masses of 

 air in the cold upper regions of the atmosphere, 

 which streaming upwards, come in contact with cold 

 currents moving laterally, or otherwise generate cir- 

 cumstances that will cause precipitation. 



A small increase of elevation compensates, in 

 adding to the quantity of rain, for a great distance 

 from the sea. At Geneva the annual fall of rain is 

 40 inches, while at Paris (300 miles nearer the sea) 

 it is only 19 inches. 



