216 ICE ITS FORMS AND FUNCTIONS. 



Tliis snow, formed of frozen vapour, and composed of my- 

 riads of the most delicate geometrical crystals, occurs during 

 winter in all the higher latitudes, and at great elevations in 

 all latitudes, wherever the surrounding air falls below the 

 average of 32. As the atmosphere frequently consists of 

 strata of different temperatures, snow may he formed in the 

 higher regions, and yet in falling may pass through a warmer 

 stratum, and he melted into rain before it reaches the earth. 

 But in ordinary circumstances it falls on the land-surface in 

 soft downy flakes ; and if that surface be at or under 32, 

 it often accumulates in great thickness, and especially in all 

 the higher and colder regions. In the lower grounds it is 

 melted and carried off by the next thaw ; in the higher 

 mountains, where it falls in dry needle-like crystals,* and 

 rarely or ever in flakes, it may endure, summer and winter, 

 for generations. But even there it cannot remain un- 

 changed ; summer suns and the pressure of newer accumu- 

 lations condense and urge it downwards first as neve^ or 

 snow-ice, and ultimately as the pure transparent ice of the 

 glacier. Here, however, it becomes ice on land, and falls to 

 be considered under a different section of our subject. 



Like ice in the atmosphere, ice on land occurs chiefly in 

 the higher latitudes and at great elevations ; though under 

 a clear sky and extreme night-radiation, ice may be formed 

 on the ground even in sub-tropical and 'tropical countries. J 



* The sands of the burning desert are not so light nor so easily moved 

 as this dry crystalline snow-powder of the loftier mountains. The slightest 

 breath disturbs it ; the storm-wind sweeps it from the exposed heights,, 

 and drifts it into the sheltered gorges in masses hundreds of feet in 

 thickness. 



f Nev$ the name given to the stratified slightly - compressed snow 

 of the higher Alps before it is condensed into the crystalline ice of the 

 glacier. 



+ The destructive effects of these night-frosts, under a clear, dry, and 

 serene sky, are now unfortunately too well known to our Australian sheep- 



