268 EVAPORATION. 



warmer one, they become capable of holding an addi- 

 tional quantity of water in solution, and therefore take 

 this additional quantity from the clouds, that come in 

 their way. 



HAIL, SNOW, ETC. 



Hail is caused by drops of rain passing through air 

 sufficiently cold to freeze them. It is said to occur prin- 

 cipally in the temperate zones. In the torrid zone it is 

 seldom or never cold enough to form hail so near the 

 earth as the clouds usually are, and should hail be form- 

 ed in the higher regions of the atmosphere, the great 

 heat which it would encounter in descending would 

 probably melt it. In the frigid zones, on the contrary, 

 the intense cold would freeze the vapor, before it had 

 formed into drops ; and during the few days of summer 

 in those zones, the rapidly increasing heat renders the 

 air capable of holding so much water in solution that 

 the weather is generally pleasant. But in the temperate 

 zones, which are exposed to currents of warm air from 

 the south, and of cold air from the north, drops of water 

 are often frozen after attaining a considerable size. The 

 coldness of air generally increases in proportion to its 

 elevation, but sometimes a higher stratum is warmer than 

 a lower one. Hail-stones frequently result from such an 

 inversion, and, as might be expected under such circum- 

 stances, are generally attended by wind. The wind, par- 

 ticularly in the torrid zone, blows from the poles towards 

 the equator. The warm air at the equator rises, and flows 

 back to restore the equilibrium. As it flows back, it 

 grows colder. If it is so full of water, that on cooling it 

 will deposit a part, the water thus deposited will pass 

 through the current of air below, moving from the poles, 

 and if this is sufficiently cold to freeze it, hail will be 

 formed. It is believed that hail is generated in the higher 

 regions of the atmosphere. Warm air is lighter than 

 cold air, and will therefore tend to rise higher. Air, that 

 passes swiftly from a warm region to a cold one, will have less 

 time to cool in passing, than if it passes more slowly, and 

 therefore will rise high, and pass on, till it comes in con- 

 tact with very cold air, before it will deposit the water 



