SNOW, a well known substance, form. ed by the freezing of the vapours in the atmosphere. It differs from hail and hoar-frost, in being as it were crystal lized, which they are not. This appears on examining a flake of snow by a mag nifying glass ; when the whole of It will appear to be composed of fine shining spicula, diverging like rays from a centre. As the flakes fall down through the atmosphere, they are con tinually joined by more of these radia ted spicula, and thus increase in bulk, like the drops of rain or hail-stones. Dr. Grew, in a discourse on the nature of snow, observes, that many parts there of are of a regular figure, for the most part stars of six points, and are as per fect and transparent ice as any we see on a pond, &c. Upon each of these points are other collateral points, set at the same angles as the main points themselves among which there are divers others ir regular, which are chiefly broken points, and fragments of the regular ones. Others, also, by various winds, seemed to have been thawed, and frozen again into irregular clusters ; so that it seems as if the whole body of snow were an infinite mass of isicles irregularly figur ed : that is, a cloud of vapours being ga thered into drops, the said drops forth with descend ; upon which descent, meet ing with a freezing air as they pass through a colder region, each drop is im mediately frozen into an isicle, shooting itself forth into several but these still continuing their descent, and meet ing with some intermitting gales of warmer air, or in their continual wattage to and fro touching upon each other, some of them are a little thawed, blunted, and again frozen into clusters, or entangled so as to fall down in what we call flakes. The lightness of STIOW, although it is firm ice, is owing to the excess of its surface in comparison to the matter contained un der it : as gold itself may be extended in surface till it will ride upon the least breath of air. The whiteness of snow is owing to the small particles into which it is divided ; for ice, when pounded, will become equally white. According to Beccaria, clouds of snow differ in nothing from clouds of rain, but in the circum stance of cold that freezes them. Both the regular diffusion of the snow, and the regularity of the structure of its parts, (particularly some figures of snow or hail which fall about Turin, and which he calls rosette,) show that clouds of snow are acted upon by some uniform cause like electricity ; and he endeavours to show how electricity is capable of forming these figures. lie was confirmed in his conjectures, by observing that his appa ratus for observing the electricity of the atmosphere never failed to be electrified by snow as well as rain. Professor Win throp sometimes found his apparatus elec trified by snow when driven about by the wind, though it had not been affect ed by it when the snow itself was falling. A more intense electricity, according to Beccaria, unites the particles of hail more closely than the more moderate electricity does those of snow, in the dame manner as we see that the drops of rain which fall from thunder-clouds are larger than those which fall from others, though the former descend through a less space.
Were we to judge from appearances only, we might imagine, that so far from being useful to the earth, the cold humi dity of snow would be detrimental to ve getation. But the experience of all ages asserts the contrary. Snow, particularly in those northern regions where the ground is covered with it for several months, fructifies the earth, by guarding the corn or other vegetables from the in tenser cold of the air, and especially from the cold piercing winds. It has been a vulgar opinion, very generally received, that snow fertilizes the land on which it falls more than rain, in consequence of the nitrous salts which it is supposed to acquire by freezing. But it appears from the experiments of Margraaf, in the year 1731, that the chemical difference be tween rain and snow water is exceeding ly small; that the latter is somewhat less nitrous, and contains a somewhat less proportion of earth than the former; but neither of them contains either earth, or any kind of salt, in any quantity which can be sensibly efficacious in promoting vegetation. Allowing, therefore, that nitre is a fertilizer of land, which many are upon good grounds disposed utterly to deny, yet so very small is the quantity of it contained in snow, that it cannot be supposed to promote the vegetation of plants upon which the snow has fallen. The peculiar agency of snow as a fer tilizer in preference to rain, may admit of a very rational explanation, without recurring to nitrous salts supposed to be contained in it. It may be ascribed to its furnishing a covering to the roots of ve getables, by which they are guarded from the influence of the atmospherical cold, and the internal heat of the earth is pre vented from escaping. The internal parts of the earth are heated uniformly to the forty-eighth degree of Fahren heit's thermometer. This degree of heat is greater than that in which the watery juices of vegetables freeze, and it is pro pagated from the inward parts of the earth to the surface on which the vege tables grow. The atmosphere being variably heated by the action of the sun in different climates, and in the same climate at different seasons, communi cates to the surface of the earth, and to some distance below it, the degree of heat or cold which prevails in itself. Dif ferent vegetables are able to preserve life under different degrees of cold, but all of them perish when the cold which reaches their roots is extreme. Provi dence has, therefore, in the coldest cli mates, provided a covering of mow for the roots of vegetables, by which they are protected from the influence of the atmo spherical cold. The snow keeps in the internal heat of the earth, which sur rounds the roots of vegetables, and de fends them from the cold of the atmo sphere.