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Page:Popular Science Monthly Volume 38.djvu/537

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place. The heat conquers, but at a great loss of its valuable supplies, and a consequent refrigeration of the adjacent waters, air, and land.

In the southern seas this effect of the snow-fall is much more considerable. A belt of glacier-forming lands surrounds the south pole, and the annual iceberg fleet is much larger than that of the north. The air indraught to the north polar region is estimated to extend over a disk of fifty-five hundred miles diameter; that to the south polar region over a disk of seven thousand miles diameter. The former is largely composed of land surface; the latter is nearly all water, and its air is therefore much more charged with moisture. In consequence, the moist air which reaches the south frigid zone is greatly in excess of that which reaches the northern zone of cold, and the snow-fall there must be very much more considerable. It is estimated that the south polar ice-cap can not be less than three miles and may be twelve miles in height. The thrust of this vast ice mountain upon the viscid material beneath it is necessarily enormous, and a lofty ice-cliff is pushed off the land at a rate of not less than a quarter of a mile annually, and this around a circle of great extent. Fortunately, the immense fleet of huge icebergs, thus annually launched, has no continental land to act upon, its refrigerating influence being mainly exercised upon stretches of ocean out of the ordinary channels of navigation, and far removed from the important seats of human habitation.

There was a time, far in the past, but within the era of man's occupancy of the earth, when the influence of the snow was enormously greater than at present, and when the atmospheric chill, stored in the falling flakes, rendered a vast region of the northern continents unfit for human habitation, and extended the border of the frozen zone far toward the present limits of tropical heat. Doubtless if at present all the snow which forms in the upper air should reach the earth's surface, a glacial epoch would now exist in the north temperate zone. The experience of balloonists and of mountain-climbers teaches us that snow forms and falls in all seasons of the year. This is melted by the warmed lower strata of air, and the earth thus saved from its chilling influence. The solar heat, which has already done good work for man upon the surface, performs new and useful labor for him in the atmosphere, by melting this falling snow, so that its water reaches the earth only in the form of rain.

At the period mentioned the snow limit in the atmosphere was much lower than at present, and the great bulk of the snowfall reached the surface unmelted. As a result, the region of an annual snow surplus extended much farther south than at present, covering much and perhaps all of British America, and a broad