Page:Popular Science Monthly Volume 15.djvu/855

Rh apsides; C and D are, respectively, the vernal and autumnal equinoxes; E F is the section of a plane passing through the poles of the earth at right angles with the plane of its orbit. It is evident that this plane will coincide with the line of the apsides once every 10,500 years; E is the winter solstice; F the summer solstice; C is the vernal equinox. While the earth is passing through B to D the north pole is inclined toward the sun. This period is the summer of the northern hemisphere.

D is the autumnal equinox. While the earth is passing from D through A to C, the north pole is inclined from the sun. This period is the winter of the northern hemisphere. In the southern hemisphere these seasons are reversed.

By consulting the diagram it will appear that the arc C B D is greater than the arc DAC. Therefore, the summer of the northern hemisphere is now longer than its winter. On the contrary, the winter of the southern hemisphere is now longer than its summer. At present, this difference is about eight days. It has been greater, but is gradually diminishing.

The southern hemisphere has at present a winter of 187 days, and a summer of 179 days. We may justly infer that during this winter more snow and ice accumulated at the south pole than the heat of the shorter summer is able to melt. The amount of this increase is very slight in a single year, but it accumulates a large aggregate in the course of ages.

This accumulation of ice at the south pole is continually increased and thickened by the deposition of moisture from the atmosphere. Every wind from warmer regions that passes over it adds to its mass.

The "Antarctic Continent" is an ice-cap, nearly circular in form, and about 3,000 miles in diameter, unexplored and uninhabitable. We can not easily ascertain its thickness. The arctic ice-cap is much smaller, and is honeycombed by the Kuro Siwa, or Japan Current, and the Gulf Stream. Nevertheless, the Greenland Archipelago seems covered with glaciers often several thousand feet in depth. If we could assign to the Antarctic ice-cap a thickness of 15,000 feet, we should have a mass of ice large enough to displace the earth's center of gravity nearly a mile to the southward of its center. A gradual displacement of this sort, caused by the slow accumulation of ice, would produce an imperceptible drainage of the oceans from the north to the south, and the gradual emergence of northern and submergence of southern continents.

If we examine the globe, we seem to discover an actual result of this sort. The greatest mass of the ocean is gathered about the south pole. The northern hemisphere includes about five sixths of the land surface of the globe.

Moreover, geologists affirm, that this inequality is increasing; they assert that the northern continents are slowly rising, and that the