474 



PHYSICAL GEOGKAPHY. 



The average annual amount of precipitation for the whole of England may be very 

 moderately estimated at 30 inches, or two feet and a half of water. According to this 

 estimate, nearly two millions of tons of water are annually discharged from the clouds 

 upon every square mile of the surface, or three thousand tons for each English acre. 

 Rain-water, when collected immediately upon its descent, apart from towns and other 

 sources of contamination, is the purest that can be obtained without having recourse to 

 distillation. 



A considerable portion of aqueous vapour in the atmosphere is frozen in the cold season 

 in extra-tropical latitudes, and the particles of ice uniting together in their descent become 

 converted into flakes of snow, and cover the surface of the earth with a mantle of the 

 purest white, stretching over bush and brake, lawn and mountain. Snow, examined with 

 the aid of a microscope, exhibits structures of exquisite beauty, regularity, and endless 

 variety, though it sometimes presents no peculiarity of form, but falls in very minute 

 globular particles. Commonly a snow-flake consists of a series of crystals formed inde 

 pendently in the upper regions of the air. These are united in groups while descending 

 through the atmosphere, by its agitations striking them against each other. The flicker 

 ing and gradual descent of the flakes is owing to their great extent of surface in comparison 

 with their volume. A number of brilliant icy spiculas, or points diverging from a common 

 centre, resembling stars having so many rays, apparently wrought with the nicest art, is 

 the usual form of the crystals, which are for the most part hexagonal, presenting a nucleus 

 with six divergences. This stelliform shape is the ordinary appearance of snow, but the 

 detail varies, as in the adjoining illustration. Dr. E. D. Clarke, speaking of the breaking 

 up of the winter season at St. Petersburg, remarks: "Snow, in the most regular and 

 beautiful crystals, fell gently on our clothes, and on the sledge, as we were driving through 

 the streets; all of them possessed exactly the same figure, and the same dimensions. 

 Every particle consisted of a wheel or star, with six equal rays, bounded by circum 

 ferences of equal diameters ; they had all of them the same number of rays branching 

 from a common centre. The size of each of these little stars was equal to the circle 

 presented by dividing a pea into two equal parts. This appearance continued during 

 three hours, in which time no other snow fell, and there was sufficient leisure to examine 

 them with the strictest attention." A microscope applied to a flake of snow will unfold 

 this mode of structure, as well as other varieties in our climate ; but it is in the polar 

 regions that snow assumes its most beautiful and varied forms. Scoresby has figured 

 ninety-six varieties, distributed into classes of lamellar, spicular, and pyramidal crystals, 



