feet in thickness, and a displacement of the earth's center of gravity one mile toward the north at the height of the glacial age. In fact, it is not necessary to assume any such amount of displacement. If the earth's center of gravity coincided with its center, so as to equalize the amount of water in the northern and southern hemispheres, Itasca Lake would not be more than 600 feet above sea-level. Now push the center of gravity 2,000 feet toward the north, and the Arctic Ocean would be so much deeper over the pole, and the water would be about 1,000 feet deeper at the latitude of 45°. To accomplish this result, we must calculate that the space within the Arctic Circle was covered by an ice-cap averaging perhaps 8,000 feet in thickness—an entirely supposable case. Such an amount of displacement would flood all the low lands of North America down to the line of 40°, and fully satisfy all the conditions of the problem.
It thus seems probable that there have been many glacial periods in each hemisphere, and that the ocean, like a mighty pendulum, vibrates from pole to pole through vast but regular periods. It is not necessary to suppose a cataclysm at the end of each period, as some of the earlier writers did; but rather, an insensible drainage of waters, which so gradually submerges the lands and pushes the human race before it, as hardly to be perceptible in the course of generations; ever uncovering new continents, and opening up fresh fields and pastures new to human industry, when the old are exhausted.
The southern hemisphere is now undergoing the slow refrigeration of its long winter. This began about 6,500 years ago; it will end about the year 4,870. It has passed its middle, but not its culmination, even as the greatest average cold of our ordinary winter is nearer the vernal equinox than the winter solstice. It is probable that, 2,000 years from now, the southern continents will be still more deeply deluged; the Antarctic ice-cap will have extended several hundred miles to the northward, and the glaciers which have already appeared among the Andes will have covered the plateaus of Patagonia and Chili. Nevertheless, we need not expect that mankind will then witness the utmost possible degree of refrigeration, because the ellipticity of the earth's orbit is now less than it has been at certain periods in the past, and will be again in the remote future.
I feel that, in this discourse, I have ventured upon doubtful and perilous ground. Nevertheless, however illogical and imperfect my conclusions may have been, I feel certain that herein is the key to the mystery. I leave the question, trusting that abler minds may be directed to its consideration and solution.
