THE GLACIAL EPOCH.
LTO THE EDITOR OF THE " SPECTATOR." J Sun,—In your review of Sir H. H. Howorth's book on " The Glacial Nightmare," in the Spectator of July 29th, you say :— " Looking over the applications of the astronomical theory of vicissitudes of climate, we cannot but have our confidence shaken by the irreconcilable difference of opinion which exists among its supporters as to its direct, and still more as to its indirect, effects. To give one example,—Croll thought that the hemisphere of cold winter would be the glaciated one ; Murphy, on the other hand, was of opinion that the hemi- sphere of cool summer would be that which is glaciated." Croll is dead, and cannot reply ; but I still think that the chief condition of glaciation is a cool summer. I am no geologist, and I am no mathematician, consequently no astronomer; but I have arrived at this conclusion by applying common-sense to simple and well-known facts of physical geography.
The extent of permanent snow on mountains is not deter- mined by the lowness of the mean or average temperature throughout the year, but by the temperature of the hottest months. This is, or ought to be, self-evident ; for permanent snow is, by definition, snow that remains unmelted during the summer. A cool summer thus produces extensive snowfields on the mountains, and far-descending glaciers in the valleys; while a cold winter has no effect whatever on the extent of these. According to Hopkins, of Cambridge, the eminent physical geologist, the mean temperature of the year in the Altai mountains of Central Asia is below freezing-point, and yet they contain no great glaciers, and the limit of perpetual snow is at a height of 6,000 ft. This, of course, is due to the warmth of the summer; but if the mean temperature of the year continued for every month unchanged, it is obvious that the whole country would be covered with perpetual snow, and the climate would be that of the glacial period. From astronomical causes, this state of constant, though not intense cold, must sometimes occur. Croll, in his " Climate and Time," states, on the authority of the American astro- nomer, Stockwell, that the earth's orbit is subject to irregular fluctuations of eccentricity, never exceeding about 7 per cent, The periods of maximum eccentricity are very long. When, during one of these periods, it occurs in the precession of the equinoxes that the earth's greatest distance from the wan is attained during the summer of either hemisphere—the Northern or the Southern—the summer of that hemisphere will be cool, the snow of winter will remain unmelted, and there will be a glacial climate. In the opposite hemisphere, on the contrary, the earth's nearest approach to the sun is attained during summer, and there is a hot summer. This also must promote the glaciation of the opposite—the glaciated —hemisphere, because the great heat will cause abundant evaporation, part of which will fall in snow on the opposite hemisphere, where it is winter. The two conditions most favourable to glaciation will thus combine a snowy winter and a cool summer.—I am, Sir, &o.,
Previous page
