Page:Popular Science Monthly Volume 5.djvu/424

408 where they were lost. The velocity of a glacier depends chiefly upon the angle of slope over which it moves. The hut of Huji moved 336 feet in a year. But the motion in different portions of a glacier is very unequal, slow at the margins and at the bottom where friction retards progress, but may attain a velocity of three feet or more in a day at its line of most rapid flow. It has been estimated that the ice-sheet which covered New England at its greatest development in the glacial age may not have advanced more than a foot in a week, a mile in a century.

A question has arisen, Do glaciers slide upon their beds? It seems to be conceded that sliding takes place to a limited extent. It may occur where the uniform flow of the glacier is interrupted, and separation of its parts produces crevasses, as along its margins, and over an uneven bed. We are chiefly concerned, however, with the motion which has its origin in the physical properties of ice.

The flowing of a glacier may be quite independent of its sliding motion, if such it has. It flows because of its plasticity, its molecules undergoing incessant change of position as they do in ice under pressure, and regelation goes on throughout the mass. By these means its cohesion and continuity are maintained.

It is often stated that the temperature of the interior of a glacier must be much below the freezing-point. This is probably an error, the temperature throughout differing but little from 32°. The pressure, indeed, may be enormous, and portions of the ice be liquefied by it, but the water which is "ice-cold" escapes through innumerable fissures, and the freezing-point is not lowered by the pressure, as it would be if the water of liquefaction did not escape. The constant flowing of a glacier necessitates unceasing supply, and its source is found accordingly in that zone of elevation where snows accumulate. The snow-fall of which the glacier is born implies vapor clouds and condensation, and equally evaporation, the proximity of a warm climate and expanse of ocean. Hence it is inferred that cold and warm climates were contiguous during the age of glaciers, as they are at this period of their decline. Glaciers relieve the land of accumulating snows as streams do of excess of waters. But for these, mountains reaching above the line of perpetual frost would become buried, and the "ocean piled upon the land." But such a process has its limitations in the economy of Nature.

The snow which falls in great volume upon mountains is a dry powdery mass, and cannot be consolidated until some liquefaction has taken place. This quickly occurs. Through the clear air of great altitudes the sun's rays fall with intense power upon objects, even while the temperature is at freezing in the shade. Portions of the surface snows are thus melted, the under portions are moistened by the percolating waters, and regelation begins. The phenomenon of the snow-ball is here reproduced on a gigantic scale, differing in this: