4o8 THE POPULAR SCIENCE MONTHLY. 



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 sepa- 

 ration of its parts produces crevasses, as along its margins, and ovei 

 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 press- 

 ure, 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 pi-obably 

 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 be- 

 come buried, and the '' ocean piled upon the land." But such a process 

 has its limitations in the economy of JNature. 



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: 



