Page:Popular Science Monthly Volume 15.djvu/856

836 of the south Pacific are sinking. It is more probable that the water is slowly draining away from the northern hemisphere and accumulating in the southern. If our hypothesis he true, this process will continue for some thousand of years to come; until, through the submergence of southern lands and the extension of the Antarctic icecap, the condition of the southern hemisphere will approximate to that of the northern continents during their glacial age. Indeed, if the present position of the earth's axis with respect to the line of the apsides were invariable, we might expect to see a final complete submergence and refrigeration of the southern hemisphere.

But this position is not invariable. As I remarked above, it is slowly changing, making a complete revolution in 21,000 years. Refer to the diagram, and you will see that when F coincides with B, the winter of the southern hemisphere is longest; when F coincides with C, winter and summer in each hemisphere are of equal length; when F coincides with A, the summer of the southern hemisphere is longer than its winter. The earth occupies successively all these situations during its great cycle of 21,000 years. Each hemisphere passes through alternate periods of a preponderance of summer or winter; each period being 10,500 years.

There is one more factor in this problem which must be considered, and that is the periodical variation in the ellipticity of the earth's orbit. Sometimes the line of the apsides is longer than at other times. This variation occupies regular but vast periods of time. It is evident that when the variation is greatest, the accumulation of ice at the winter pole must be most rapid.

The 10,500 years during which the pole is refrigerated and deluged, may be called its great winter; the other half of the cycle, its great summer.

Unless astronomical calculations fail, the last great summer of the northern hemisphere commenced some 6,500 years ago. When it began, northern America, Europe, and Asia were frozen and deluged. The Arctic Ocean extended to a line south of the present bed of the Great Lakes. The Alps and the Alta were also southern boundaries of this ocean. Europe was the home of a swarthy, dwarfish race, who hunted the aurochs and great hairy mastodon at the foot of the glaciers that then half overflowed the continent.

But the intenser suns of many ages have done their work. The glaciers have melted, dwindled, retreated to the high Alpine valleys, and to the high northern latitudes. The swarthy troglodytes and lake dwellers have followed them. Under the name of Esquimaux, Lapps, and Finns, a feeble remnant of these preadamite people still lingers within the Arctic Circle; but their doom as a living race seems near and sure. The Aryans have come marching across the Eastern steppes, for the building of society, civilization, human history.

In the outset of this argument, I assumed an Arctic ice-cap 15,000