Page:Popular Science Monthly Volume 8.djvu/727

Rh our winters are short and mild, and our summers long and sunny. During the cycle which shall comprise the latter case, our winters will be rigorous and our summers short. The northern hemisphere is now having its great summer. In about 10,000 years it will be in the midst of its great winter; and whatever differences there may be between the two hemispheres, owing to astronomical causes, will then be in full force against the northern.

A distinguished Scotch mathematician, Mr. James Croll, has estimated that the melting of a mile in thickness of the present antarctic ice would raise the sea-level at the north pole 300 feet, and at Glasgow 280 feet. We have calculated, from data which were intended to be under-estimates in every case, that there were at least two and a half miles of average thickness in what geographers call the great ice-cupola of the south pole. If, therefore, not only this were removed, but an equal quantity of ice were deposited at the north pole, there would be a deepening of the sea at the arctic circle of 1,500 feet.

Thus it is seen that, as certainly as terrestrial revolutions continue, in the course of 10,000 years there must come an entire reversal of polar conditions. The southern waters must be drained off to make the oceans of an opposite hemisphere. New lands, enriched with the sediment of a hundred centuries, will rise up to extend the borders of the old south continents, and islands joining together, will expand into mainlands. At the same time the northern continents must be in great part submerged, and their summits and ranges become the bleak islands and the bold headlands of a tempestuous ocean. Central Asia, with its broad table-lands, may still retain the name of a continent; but, beyond a few outlying islands, there will be no Europe, and but little of North America left. The Atlantic waters will stand five hundred feet over Lake Superior, and will wash the base of the Rocky Mountains in all their length. A new Gulf Stream may again, as it must often have done before, flow up the valley of the Mississippi, returning the deltas to the prairies, and remaking the beds of the garden of the world. These are no idle or impossible fancies. Not only are they the results of rigorous calculation, but they accord perfectly with the unmistakable evidences which the ocean has left, all over our land, of its recent work and presence.

The time-honored geologist, Sir Charles Lyell, lays great stress on the quantity of land and the configurations of continents, as chiefly efficacious in the great climatic changes. But it may be pertinently asked, What becomes of his continents and configurations when the seas of one pole advance to the other, as they unquestionably do, as they cannot but do, every 10,500 years, obedient to the transfer of