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Page:Popular Science Monthly Volume 32.djvu/693

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673
OUR ICE-SUPPLY AND ITS DANGERS.

downward inch by inch. But this condition of affairs, quite ideal from the standpoint of the ice-farmer, is apt in this region to be evanescent. If the grip of the cold relaxes by day, the formation of ice may stop, and even a film of that already made may melt away in the water beneath; but at night again another layer may be added, and so, with many halts, retreats, and slow advances, little by little the ice mass thickens. But who would imagine that, written in the ice, as plainly as the sequence of geologic ages is written in the rocks, is the record of these alternate victories of heat and cold as they contended for the mastery of the water during the winter days and nights? Strange as it may seem, the record is there, however, and, stranger yet, is written in air. Look at the edge of a cake of ice which has formed in comparatively still water during such alternations of temperature as are common in our winters, and you will be very apt to see a series of bands of transparent ice, between which lie layers of tiny air-bubbles. In still water, when the ice for any reason stops forming for a time, bubbles of air from the water or from the bottom are apt to rise and collect beneath the ice, and when the freezing again begins they are entangled and held fast between the old and the new ice-layers, a permanent record of the relaxation of the thrall of the cold long enough for their collection. In running water such bubbles are apt to be swept away, and the ice remains transparent.

While the ice is thus forming the ice-farmer looks on, his spirits rising in inverse ratio to the height of the thermometer. To the vagaries of the temperature he must reconcile himself as best he may. But let his bête noire, the snow—if so violent an antithesis be permissible—appear, and he will be on the alert at once. The snow-flakes, delicately adjusting themselves to one another as they settle down upon the ice, build up among their crystals myriads of tiny air-cavities, and the whole forms a veritable blanket which hinders radiation. It is warm for the same reason that a down comfortable is—it prevents the escape of heat. Now, what shall our ice-farmer do? It does little good to swear at the snow, although he usually has recourse to this procedure first. If the already formed ice is thick enough to bear the teams, he may scrape the snow off, and then the freezing can go on. But if not, he sends his men over the field to cut small holes here and there through it; the water wells up, flows over the top, forming a layer of slush, a good deal of the air is expelled, and the whole freezes, forming a whitish layer which is called snow-ice. This layer is whitish because of the air-bubbles which it still retains, but it conducts off the heat fairly well, and his crop goes on forming. This operation is called "tapping" or "bleeding" the ice. Ice which has a very thick snow layer is called "fat ice." This snow-ice is not as valuable as clear ice, for householders object to it because they fancy that it is not so pure, and the assurances of the dealers that the impurity is only air appear to have little weight. So the more responsible dealers usually find it