Page:Popular Science Monthly Volume 38.djvu/533

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HERE are two processes constantly active upon the surface of the earth which are of the utmost importance as regards its suitability for human habitation—the storage of heat and the storage of cold. Of these we are here concerned only with the latter. The source and method of the storage of cold (a negative process, which we may here treat as a positive) are much less evident and not so generally known as those of heat-storage, and a review of them may be of interest.

The source of the stored cold is the upper atmosphere, and the principal storing substance snow. Here we are on ground familiar only to scientists. Readers generally are not aware of the vitally important part which snow plays in the economy of nature. The lightly falling snow-flake, with its poetic affiliation and its attractive aspects, has its aspect of terror as well, for the feathery snow has done more to limit man's dominion of the earth than any other of the unfriendly agencies of Nature, even if we count the fiery ravage of the volcano and the ruinous work of the earthquake. While the rains are friends to man, and efficient agents in the progress of civilization, the snows are his enemies, and the most persistently hostile of his foes.

It need scarcely be said that the invigorating beams of the sun visit the earth in very differing measure, varying from tropical profusion to frigid sparseness. This diversity of heat distribution is partly overcome by the agency of the winds and waters, particularly the latter, since the great ocean currents carry vast supplies of heat from the torrid zone toward the poles, and drive far backward the boundaries of the realm of frost. The agency of the air in this heat convection is of less importance. The anti-trade winds move through the upper atmosphere, and lose their heat before descending to the earth; but surface winds from the tropics convey a considerable share of the torrid heats to the colder zones.

Snow is the great opponent to the full effect of this distributed heat. It constitutes an agent of Nature by which the chill of the upper atmosphere is conveyed to the earth's surface, and stored there in a more or less persistent form, which requires much of the solar heat and the warmth of tropic winds and waters to overcome. If it be asked how snow can produce such an effect, we must advert to the heat relations of water. A large supply of insensible heat—latent heat, as it is called—exists in liquid and gaseous matter. In the freezing process this heat becomes