For every 10 inches of rain which fall at the following places in winter, there fall in summer respectively, 8} inches in the west of Great Britain, 11 inches in the east of Great Britain and west of France, 15 inches in the east of France, 20 inches in Germany, and 27 inches in the north and east of Russia.

338. The peculiarity of the rainfall of the basin of the Mediterranean depends on (1) its proximity to the burning sands of Africa, (2) a predominance of northerly winds resulting chiefly from that position, and (3) the Pyrenees and Spanish sierras to the west, on which the south-west winds precipitate their rains before arriving on the north shores of the Mediterranean. In the valley of the Rhone, four times more rain falls in autumn than in summer ;

and south of the Alps, six times more rain falls with north-east than with south-west winds, being the reverse of what takes place in England. In Italy the quantity of rain diminishes as we approach the south, because south winds get wetter, and north winds get drier as they proceed on their course. Along the Syrian and North African coasts it rarely rains in summer, but frequently in winter. In the valley of the Rhone, the annual fall ranges from 20 inches on the coast to 63 inches at St Rambert, the average being 30 inches. This is also the average of the valley of the Po; but on ascending the long slopes northward to the Alps, it rises, as at Tolmezzo, to 96 inches.

339. Rainfall of America.—The rainfall in the west of the American continent is distributed similarly to that of Europe the quantity being chiefly dependent on the physical configuration of the surface over which the westerly winds blow. In North America the yearly amount increases as we proceed northward ; thus at San Francisco it is 22 inches ; at Fort Reading, 29 inches ; at Fort Oxford, 72 inches ; at Fort Vancouver, 47 inches ; at Astoria, 86 inches ; at Steilacoom (Wash. Ter.), 54 inches; and at Sitcha, in Russian America, 90 inches.

340. But in the United States the manner of the distribution of the rainfall differs greatly from that of Europe. The United States are dependent for their rain, not on the Pacific Ocean, but on the Gulf of Mexico. The high range of the Rocky Mountains in Central America plays an important part in the rainfall ; for these mountains present a barrier to the passage of the tradewinds which blow across the Gulf of Mexico, which are therefore, partly on this account, and partly on account of the heated plains of the States, turned or drawn to the northward, and spread themselves over the States, especially over the low basin of the Mississippi. Thus, then, the greatest part of the moisture will be drawn into the valleys where the heat is greatest, and the least

;

part into the high mountainous regions, where respectively it will be disengaged and fall in rain. If this be the case, then the greatest quantity will fall in the valleys, and the least on the higher groundsma mode of distribution the opposite of what obtains in Europe. That such is the case the following remarks by Mr Blodget on the rainfall of America, given in the 'Army Meteorological Register,' will show :-“For much the larger area of the United States, and for all portions east of the Rocky Mountains, the distinguishing feature of the distribution of the rainfall is its symmetry and uniformity in amount over large areas. The quantity has rarely or never any positive relation to the configuration of the surface, which would identify it with Europe and the North Pacific coasts; and in contrast with these it has a diminished quantity at the greater altitudes generally, and the largest amounts in the districts near the sea-level. It also differs from these districts, and from large land areas generally, in having a larger amount in the interior than on the coast, for the same latitude, at least as far north as lat. 42°.” The rainiest districts are Florida, the low flats of the Mississippi, then along the course of its valley, then in Iowa, that remarkable depression at the head of the river; and the least quantities on the Alleghanies, especially their 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,

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 IowaFort 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.

341. 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.

66;

Snow.

342. 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, usually in the form of six-pointed stars. About 1000 different kinds of snow-crystals have been already observed, and many of them figured by Scoresby, Glaisher, and others. These numerous forms have been reduced to the fol

lowing five principal varieties. 1. Thin plates-the most numer-
ous class—comprising several hundred forms of the rarest and

most exquisite beauty (figs. 27 to 33). 2. A spherical nucleus
or plane figure, studded with needle-shaped crystals (fig. 34).

3. Six or more rarely three-sided prismatic crystals (fig. 35).
4. Pyramids of six sides (fig. 36). 5. Prismatic crystals, having
at the ends and middle thin plates perpendicular to their length
(fig. 37). The forms of the crystals of the same fall of snow are

Figs. 34
35
36

37
generally similar to each other. Snow-flakes

Snow-flakes vary from an inch to 0.07 nch in diameter, the largest being observed when the temperature is near 32°, and the smallest at very low temperatures.

343. The crystals of hoar-frost being formed on the leaves of trees and other bodies which greatly modify the temperature, are on this account not so regularly formed, and are more opaque. It has been observed that each tree or shrub is frequently covered with crystals peculiar to itself.

344. 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.

345. 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 America 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 III.), 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.

346. The white colour of snow is caused by the combination of the different prismatic rays which issue from the minute snowcrystals. 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, these different colours blend into homogeneous white. Pounded glass and foam may be cited as additional 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.

347. Red snow and green snow have been occasionally met with in the arctic regions and in other parts of the world. These colours are produced by the presence of microscopic organisms, about lobo inch in diameter, which grow and flourish in the region of eternal snow. It is called Protococcus nivalis.

348. 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, 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 regi fons which would otherwise remain barren wastes.

C

349. 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 heavier. 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. This, however, is only a rude way of comparing the snow with the rainfall, owing to its varying compactness. It is 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, and then measuring the depth of the water for the rainfall.

350. On the 5th March, 1862, about 9 P.M., the Rev. Charles Clouston, Sandwich, 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, and 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 circumference. 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 6} 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."

351. Sleet appears to be formed from snow-flakes falling through a stratum of moist air at a temperature of 32° or higher. The size of the flakes is caused by the snow particles coming against