Snow is the frozen moisture which falls from the atmosphere when the temperature is 32° or lower. It is composed of crystals, usually in the form of six-pointed stars, of which about 1,000 different kinds have been already observed, and many of them figured, by Scoresby, Glaisher, and others. These numerous forms have been reduced to five principal varieties: Thin plates, the most numerous class, containing several hundred forms of the rarest and most exquisite beauty; spherical nucleus or plane figure studded with needle-shaped crystals; six or more rarely three-sided prismatic crystals; pyramids of six sides; prismatic crystals, having at the ends and middle thin plates perpendicular to their length. The forms of the crystals in the same fall of snow are generally similar to each other. The crystals of hoar-frost being formed on leaves and other bodies disturbing the temperature are often irregular and opaque; and it has been observed that each tree or shrub has its own peculiar crystals. Snowflakes vary from an inch to 7-100ths of an inch in diameter, the largest occurring when the temperature is near 32°, and the smallest at very low temperatures. As air has a smaller capacity for retaining its vapor as the temperature sinks, it follows that the aqueous precipitation, snow or rain, is much less in polar than in temperate regions. The white color of snow is the result of the combination of the different prismatic rays issuing from the _minute_ snow-crystals. Pounded glass and foam are analogous cases of the prismatic colors blending together and forming the white light out of which they had been originally formed. It may be added that the air contained in the crystals intensifies the whiteness of the snow. The limit of the fall of snow coincides nearly with 30° N. lat., which includes nearly the whole of Europe; on traversing the Atlantic, it rises to 45°, but on nearing America descends to near Charleston; rises on the west of America to 47°, and again falls to 40° in the Pacific. It corresponds nearly with the winter isothermal of 52° Fahr. Snow is unknown at Gibraltar; at Paris, it falls 12 days on an average annually, and at St. Petersburg 170 days. It is from 10 to 12 times lighter than an equal bulk of water. From its loose texture, and its containing about 10 times its bulk of air, it is a very bad conductor of heat, and thus forms an admirable covering for the earth from the effects of radiation--it not infrequently happening, 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 summer carries fertility into regions which would otherwise remain barren wastes.