GLACIAL EPOCHS.

Mataura Ensign, Volume 11, Issue 793, 27 July 1888, Page 2

GLACIAL EPOCHS.

Let us puppose the earth described a ■circle around the sun. Then we should know neither the torrid heat of summer nor the rigours of winter. A perpetual spring would reign over the entire surface of the planet, and a uniform meteorology w.mld but insensibly modify the terrestrial relief. The organic kingdom, never being trammelled in its flight, would reach the two poles. Sure of the morrow, the human race, in the normal course of its destiny, would experience neither exhaustion nor periods ofarreßt. Unfortunately, the elliptical nature of thejtellurian axis,and]thejlaws of celestial mechanics have not countenanced such Edeuic perspectives since the time that the terrestrial surface, completely cooled, began to be heated only by con tact with the sun's rays. A long and vigorous winter prevailed alternately over each hemisphere every 21,000 years. The, immense cap of neve which extended from the pool to temperate latitudes profoundly modified the relief of the earth in consequence of the erosive action exerted by glaciers upon the sides of mountains and in the valleys. Plants and animals' receding before the terrible enemy, fled towards the tropics. This hegira of the living world, carrying along man, each time shifted the axis of civilisation. The causes that bring about glacial phenomena are various Some depond upon astronomy, and others upon meteorology. The first, and most important, is the obliquity of the ecliptic —that is to say, the angle made by the , axis of the earth with the perqendicular to the plane of the orbit. This angle varies between 34° 35' 38 and 21° 58' 38" , and is now 23° 27' 50". The immediate effect of such inclination is to expos each pole alternately to an insolation of six months and to a night of the same length of time. The great depression in temperature caused by so long a uight is revealed by an immense stratum of circumpolar' ice, which the uninterrupted action of the sun's r.iys during the succeeding months can makei recede, but cannot make entirely disappear. It is a glacial period of half a year, circumscribed over a space of relatively small extent, which increases or decreases according as the world's axis approaches or recedes from its maximum amplitude. — Scientific American.