The Cause of an Ice-Age. By Sir Robert Ball, LL.
D. (Kegan Paul, Trench, and Co.)—The " cause " Sir Robert Ball finds in astronomical facts. The attraction of the planets deflects the earth from its normal orbit. The most potent of these forces is that of Venus ; next to this planet comes Jupiter, exercising about half the amount. All the planets do something. The smallest of the asteroids—Eudora by name, " 217 " by number— exercises an attraction which would snap the strongest cable as if it were a piece of pack-thread. But it is Venus and Jupiter that chiefly bring about the Ice-Age. They change the earth's orbit from a circle to an ellipse. When it is a circle we have a genial, when it is an ellipse we have a glacial age. Of course, it must be remembered that the state of things in one hemisphere is the converse of what it is in the other. These, and a variety of cognate considerations and details, Professor Ball sets out in a most lucid and interesting style.
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