CONDITIONS OF CONTINENTAL ICE. 239 



velocity ten times greater. Mr. Aniuncl Helland, for 

 example, found that the glacier of Jacobshaven has a 

 velocity of about 20 metres per diern, which is up- 

 wards of 4 miles annually. The exceptional high 

 velocity of the Greenland glaciers is no doubt owing 

 to the fact that the ice-sheet covering that continent 

 has to force its way through comparatively narrow 

 outlets. If the sheet moved off the land in one un- 

 broken mass, like the Antarctic sheet, its rate of 

 motion would be much less. 



It is the immense extent of the Antarctic continent 

 which demands such a high velocity to get rid of the 

 ice. To enable it to discharge the annual amount of 

 ice, either the sheet must be excessively thick or its 

 rate of motion excessively great. If, for example, the 

 ice were only 700 feet instead of 1400 feet thick, its 

 motion would require to be half a mile annually in 

 order that the 6 inches of ice should be got rid of ; 

 while, if it were only 100 feet in thickness the rate of 

 motion would need to be 3J miles per annum. 



It is this difficulty in getting away which is the 

 chief cause of the enormous accumulation of ice on the 

 Antarctic continent. And it is just this great thick- 

 ness in the interior that enables the sheet to get rid of 

 its superabundant ice. This is effected in two ways: — 

 1st. The greater the thickness of the ice in the interior, 

 the greater is the force by which it is impelled out- 

 wards, and, other things being equal, the greater is the 

 velocity of the ice. 2nd. The thicker the sheet becomes, 

 the greater is the quantity discharged corresponding 

 to a given velocity. The velocity being the same, the 

 quantity discharged is in proportion to the thickness 

 of the sheet. 



With the present rate of snowfall on the Antarctic 

 continent it is physical!}' impossible that the ice can 



