SNOW is the name given to the watery vapour in the upper region of the atmosphere, when frozen dur ing its descent to the earth.
Snow is a congeries of an immense nt•mber of se parate and transparent crystals of ice; and its white ness is owing to the same cause as the whiteness of froth or of painted glass, namely, to the accumulated light which each separate crystal reflects to the eye of the observer.
The specific gravity or density of snow is very va riable, as the following table will show.
When the snow falls (luring frost the flakes are al ways less, and they are greater when the air is warm. In July 1819, in the neighbourhood of Edinburgh, a slight shower of snow fell, in which the flakes were fully two inches and a half in length. Two or three years afterwards a similar fall was noticed in another part of Scotland.
Snow is occasionally found in North America in balls and in cylinders. On the 1st April 1815, Profes sor Cleaveland observed a great number of balls of snow from 1 to 15 inches in diameter, the small ones being nearly spherical, and the larger ones somewhat oval. Their texture was homogeneous, and they were extremely light, being composed of minute prisms of snow irregularly aggregated. These balls were form ed by having been rolled through a considerable dis tance by the wind, their paths upon the snow being, in general, distinctly visible. The smaller balls, how ever, were decidedly formed in the atmosphere, as they occurred in woods and in small enclosures. See Professor Silliman's Journal, vol. vi. p. 169.
Cylinders of snow were first observed by the Rev. D. A. Clark in Morris county, New Jersey. When a deep snow was on the ground a shower of rain fell, and in consequence of a sudden cold the rain was congealed on the surface of the snow, and formed up on it a cake of ice. Another shower of snow fell to the depth of 3-4ths of an inch, and the sky having suddenly cleared, the cold became very intense, and the wind blew a gale. Nature, says M. Clark, now began her sport. Particles of snow would move upon the icy crust from 12 to 20 inches, and would then begin to roll, making a track upon the ice shaped like an isosceles triangle. The balls enlarged accord ing to circumstances, and aided by the declivity of the ground, the rolls were of the size of a barrel, and some even larger. Thus the whole creation, as far
as the eye could see, was covered with snow balls dif fering in size from that of a lady's muff, to two and a half or three feet in diameter, hollow at each end to almost the centre, and all as true as so many logs of wood shaped in a lathe.
In 1812 or 1813 Mr. Hitchcook observed at Deer field, Massachusetts, similar cylinders of snow; none of them, however, were more than six or seven inches in diameter. See Professor Silliman's Journal, vol. ii. p. 132 and 375.
In the Arctic Regions, as Mr. Scoresby informs us, it snows nine days out of ten in the months of April, May, and June. With southerly winds near the bor ders of the ice, or where moist air blowing from the sea meets with a cold breeze from the ice, the hea viest falls of snow occur. In this case a depth of two or three inches sometimes falls in an hour. These heavy falls frequently precede sudden storms.
The crystals of snow present an endless variety of forms. Descartes and Dr. Hook seem to have been among the first who observed and delineated the figures of the crystals.* Dr. Green and Di'. Langwell likewise observed them.f Dr. Stocke has delineated several beautiful forms or snow which fell at Middle burg in Zealand, in 1740 and 17424 The most ele gant delineations, however, of' the particles or snow are those which were executed by Dr. Nettis of Mid dleburg, to the number of 80, and given in the Philo sophical Transactions, 1755, p. 614, and those which were observed by Mr. Scoresby in the Polar seas, given in his "Account of the Arctic Regions," to the amount of 96. The general size of the particles which exhibit these regular figures, is from one-fifth to one twentieth of an inch. We should have copied several or the figures given by Dr. Nettis, but we prefer tak ing those of Mr. Scoresby, because he has described the magnitude or each particle which he has delineat ed, and the state of the barometer, thermometer, and weather, at the time when it fell. Mr. Scoresby arranges the various modifications and crystals which he nas observed under five kinds.