=Rain Water Is Not Pure.= Hailstones often incase foreign matter that has been carried upward by violent winds. Rain water is pure when it is condensed, but it gathers other matter as it falls, such as the pollen of plants, and the broken siliceous shells of microscopic life carried by winds of the tropics; it also washes ammonia from the air in small quantities,—about thirty pounds per acre in the eastern half of the United States each year. A raindrop increases in velocity as it falls until the resistance of the air becomes just equal to the weight of the drop; after that it falls at a uniform rate. It will surprise many to learn that if it were not for the retardation effected by the resistance of the air, a raindrop falling from only half a mile would be as dangerous to life as a rifle bullet, for the speed with which a projectile travels can be made sufficient to compensate for its softness or yielding qualities. =How Much Water Is It Possible to Precipitate from the Earth’s Atmosphere?= If the entire amount of water vapor present in the atmosphere were precipitated instantly it would furnish a rainfall of only two inches for the whole surface of the earth. A steady downpour for twenty-four hours usually amounts to some two or three inches. Over small areas and in exceptional cases as many feet have been known to fall in that time, as fresh, vapor-bearing winds steadily blew into a storm center, rose, discharged their burdens as they cooled with ascent, and then flowed away, again to be charged with moisture when they came into contact with wet surfaces. It is impossible to drown the entire earth with rainfall, no matter how long continued. [Illustration: FIG. 31.—SNOW CRYSTALS.] =Snow.= Snow is water vapor condensed in the congealed form, without passing through the liquid state. When the minute pieces of ice of which the flake is composed are magnified several hundred times they are found to be composed of the most wonderfully beautiful figures. Thousands have been photographed, but the versatility of nature is so great that no two ever have been found that were exactly alike. Figure 31 gives some idea of their infinite variety and perfect symmetry. They are always governed by the number six. The most common form at the beginning of winter is a six-rayed star, each ray branching. As the winter advances and the cold becomes more severe, the flakes take a simpler form, finally becoming slender six-sided prisms with sharp ends, under the influence of severe cold waves. Great pain is inflicted on the exposed parts of the body when these prisms are encountered in a high wind.