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higher parts, and the high grounds of the Missouri district. The following figures give the annual amounts in inches at different places :-In Florida—Pensacola, 57; Fort Brook, 55; and Fort Pierce, 63: in Alabama-Monrosville, 66 ; and Mobile, 64: in Mississippi-Natchez, 58 ; and Jackson, 53 : in Louisiana-Rapides, 63 ; and New Orleans, 52: in Tennessee-Nashville, 53: in Georgia ---Savannah, 48: in Iowa-Fort Madison, 50. At Athens, in Georgia, south of the Alleghanies, the amount is 36 ; at Alexandria, in Virginia, also, 36 ; and at Jefferson, in Missouri, 38.
416. In the Northern States the quantity diminishes at most places to between 27 and 45 inches, and the mode of its distribution becomes assimilated to that of Europe.
417. Snow is the frozen moisture which falls from the clouds when the temperature is 32° or lower. The particles of which snow is composed are crystals, which are usually in the form of six-pointed stars. About 1000 different kinds of snow-crystals have been already observed, many of which have been figured and described by Scoresby, Glaisher, Lowe, and others. These numerous forms have been reduced by Scoresby to the following five principal varieties. 1. Thin plates—the most numerous class—comprising several hundred forms of the rarest and most exquisite beauty (figs. 32 to 38).
matic crystals (fig. 40). 4. Pyramids of six sides (fig. 41). 5. Prismatic crystals, having at the ends and middle thin
plates perpendicular to their length (fig. 42). The forms of the crystals of the same fall of snow are generally similar to each
other. Snow-flakes vary from an inch to 0.07 inch in diameter, the largest being observed when the temperature is near 32°, and the smallest at very low temperatures.
418. The crystals of hoar-frost being formed on the leaves of trees and other substances which greatly modify the temperature, are on this account not so regularly formed, and are more opaque. During the great frost of Christmas 1860, Henry Stephens made the observation, that each tree or shrub was covered with crystals peculiar to itself.
419. Since the capacity of air for retaining its vapour is diminished as the temperature sinks, it follows that the aqueous precipitation, snow or rain, is much less in polar than in temperate regions.
420. The limit of the fall of snow at any time of the year coincides nearly with 30° N. lat., which includes almost the whole of Europe. On traversing the Atlantic this line rises to lat. 45°, but on nearing the American continent it descends to near Charleston in lat. 33°; it rises in the west of America to lat. 47°, and again falls to lat. 40° in the Pacific. It corresponds nearly with the winter isothermal of 52° (Plate V.), because in places where the mean winter temperature is no higher than 52°, the air may be expected to fall during the coldest months occasionally to 32° or lower. Snow is unknown at Gibraltar ; at Paris it falls 12 days on an average annually, and at St Petersburg, 170 days.
421. The white colour of snow is caused by the combining of the different prismatic rays which issue from the minute snow-crystals. When the crystals are looked at separately, some appear red, others green, purple, and, in short, all the colours of the spectrum, as these are seen in ordinary crystal gasaliers; but when a mass of snow is looked at, the different colours blend into homogeneous white. Pounded glass and foam may be cited as other illustrations of the prismatic colours blending together and forming the white light from which they had been originally produced. It may be added that the air contained in the crystals intensifies the whiteness of the snow.
422. Red snow and green snow have been occasionally met with in the arctic regions and in other parts of the world. These colours are due to the presence of microscopic organisms, called Protococcus nivalis, about Todo inch in diameter, which
grow and flourish in the region of eternal snow. 423. The uses of snow are very important; thus, from its loose texture, and from its containing about ten times its bulk of air, snow is a very bad conductor of heat; and thus is an admirable covering for the earth in preserving it from the effects of its own radiation. It not unfrequently happens in times of great cold, that the soil is 40° warmer than the surface of the overlying snow. The flooding of rivers from the melting of the snow on mountains in spring and summer, carries fertility into regions which would otherwise remain barren wastes.
424. Snow is generally from 10 to 12 times lighter than an equal bulk of water; but rare cases have occurred where it was only eight times lighter. Hence in measuring the snowfall, in order to add it to the rainfall, the rule commonly adopted is to measure the depth at a place where it is about the average depth of the district, and take one-tenth as the equivalent of the rainfall. Thus, if the average depth of snow fallen be 5 inches, this would equal 0.5 inch, or half an inch, of rain; if 12 inches of snow, it would be 1.20 inches of rain, &c. This, however, is only a rude way of comparing the snow with the rainfall, being liable to considerable error owing to the varying compactness of the snow. It may be accurately measured by thrusting the open end of a cylindrical tin vessel down through the snow to the ground, and melting the snow which it brings up. The depth of the water or melted snow is considered as the rainfall.
425. On the 5th March 1862, about 9 P.M., the Rev. Dr Charles Clouston, Sandwick, Orkney, observed the snow which was then covering the ground to be rolled up by the wind into masses increasing in size as they moved before it, which were blown backwards and forwards in the eddy-wind of the house. The same phenomenon had occurred in February 1847, and was then described by him in the following terms: “ On examination, the masses were all found to be cylindrical, like hollow fluted rollers, or ladies' swan-down muffs, of which the smaller ones reminded me, from their lightness and purity, but most of them were of much greater dimensions and weight than any lady would choose to carry, the largest measured being 31 feet long and 7 feet in circumfer
The weight, however, was not so great as might have been expected from the bulk; so loose was the texture, that one 3 feet long and 61 feet in circumference was found, on being weighed, to be only 64 lb. ; the centre was not quite
hollow, but in all there was a deep conical cavity at each end, and in many there was a small opening through which one could see, and by placing the head in this cavity in the bright sun, the concentric structure of the cylinder was quite apparent. They might occupy 400 acres, and I counted 133 cylinders in one acre. A combination of favourable circumstances is required for their formation - viz., a recent fall of loose snow-flakes in calm weather, as took place on the day previous; a temperature near the freezing-point, so as to give adhesion to the snow, while it is not so warm as to thaw it; and a good breeze of wind to spring up when the other circumstances are favourable to their formation.”
426. Sleet appears to be formed from snow-flakes falling through a stratum of moist air at a temperature of 32°, or higher. The great size of the flakes is caused by the snow particles uniting by regelation as they come against each other; and they are no doubt further enlarged by the condensation of vapour on their surfaces as they float down through the moist air. Sleet falls chiefly in winter and in spring, and is very rarely an accompaniment of storms.
427. Snow-Line. — The snow-line marks the height below which all the snow that falls annually melts during summer ; above this imaginary line lies the region of perpetual snow. No general rule can be laid down for the height of this line, owing to the many different causes by which it is determined: these are, (1) the exposure of the slope of the mountain to the sun's rays, and hence, other things being equal, it is higher on the south than on the north side of mountains ; (2) the situation with respect to the rain-bringing winds ; (3) the steepness of the slope ; and (4) the dryness or wetness of the district. Hence the snow-line can only be ascertained from observation. The following are the observed heights in feet, in different parts of the globe :