of land, and in the southern, so far as is known, about 16,000,000 square miles. Now, the great problem in physical geography is. What is there in the southern hemisphere to counterbalance this great excess of land in the northern?
Humboldt has estimated that, if the mountains and highlands of Asia were leveled down and made to fill up evenly the low places, the whole continent would have a uniform height of 1,150 feet above the sea. In like manner, South America would have a height of 1,130 feet; North America of 750 feet; and Europe of 670 feet. The average of the whole he estimates at 920 feet. Of the mainlands not included in the above—namely, Africa, Australia, the polar lands, and islands—about as much is north as south of the equator. So that we may safely estimate that there is in the northern hemisphere an excess of 28,000,000 square miles of land, of the average height above-mentioned, to be counterpoised by something yet to be found in the southern hemisphere.
If there is an excess in the quantity or bulk of water south of the equator over that north of it, then the difference of weight between this excess and so much land, which is about in the proportion of one to two and a half, must be added to the unknown quantity which we are soon to look for above the southern seas. As there is, of course, the same excess of water-surface south of the equator that there is of land-surface north of it, and as we may very safely assume that the oceans have a mean depth of at least 3,220 feet (312 X 920) and that the southern waters average as deep as the northern, it follows that our unknown quantity is at the very least doubled by the above considerations. We have, therefore, to seek in the southern hemisphere what will balance 28,000,000 square miles of land at least 1,840 feet high.
We look over the map of the world, and down near the bottom we find some uncertain landmarks with many breaks, but on the whole tracing out very nearly the antarctic circle, and indicating that there is, covering nearly all that zone, an unexplored and scarcely discovered country. This impenetrable region is estimated to be as large as the continent of North America, about 8,000,000 square miles. A very little arithmetic will now prove the bold claim which I here make, that, even supposing the whole of this region to be land of the average continental height, there is still required over it all an average thickness of two and a half miles of solid ice to make the southern hemisphere equal the northern in weight.
This result of calculation is well confirmed by the information which all southern navigators have brought back from those most desolate and ice-bound regions. The zone of the antarctic has been encroached upon only in a small space south of the Pacific. On every other side, so far as has been discovered, mountains of ice block the way on and near the polar circle, which seems to be the great ice-bar-
