GLACIERS. 679 



winter is requisite ; for otherwise the snows will be melted to the same 

 line throughout the year, and will not descend much below the line of 

 perpetual congelation. 



The level of the snow-line, or that below which the snow annually 

 precipitated melts away during the year, and the distance to which 

 glaciers descend, depend mainly on the mean temperature and moisture 

 of the region, and especially the mean temperature of summer as con- 

 trasted with that of winter. The height of the snow-line on the north 

 side of the Alps is about 8,000 feet, and on the southern side about 

 8,800 feet. Below this limit, the glaciers descend 4,500 to 5,300 feet. 



The snow-line in the Pyrenees is 8,950 feet above tide level; in the Caucasus, 10,000 

 to 11,000 feet; on the south side of the Himalayas, 12,980 feet, and on the north, 

 16,620 feet; at the equator, in the Andes, 15,980 feet; in Bolivia, 18,520 feet in the 

 ■western Cordillera, and 15,920 in the eastern; in Mexico, 14,760 feet; in Chili, near 

 Santiago, 12,780 feet; in Norway, 5,000 feet in its middle portion, and 2,300 feet at its 

 northern extremity; in Kamchatka, 5,200 feet; in Alaska, 5,500 feet. 



The lower limit of a glacier sometimes varies several miles, in the 

 course of a series of years. A succession of moist years increases 

 the thickness of the glacier, and thereby its tendency downward ; 

 while dry years have the reverse effect. If the moist years have also 

 long, hot summers, the descent and lengthening of the glacier will be 

 further promoted, — since glaciers move most rapidly in summer. 

 But hot, dry years would shorten it, by diminishing the ice, and espe- 

 cially at the lower end. 



Lowering the mean temperature of a place, by cooling the summers, would lower the 

 glacier-limit. Great Britain and Fuegia are in nearly the same latitude; and yet, in 

 Fuegia, the snow-line is only 3,000 feet above the sea. If, by any means, the climate 

 of Great Britain could be reduced to that of Fuegia, it would cover the Welsh and Irish 

 mountains with glaciers that would reach the sea, the snow-line being but 1,000 to 2,000 

 feet above it; and the same cause would place the snow-line in the Alps at 5,000 to 

 6,000 feet above the sea, instead of 9,000. This change of temperature involves a 

 removal of tropical sources of heat, or an increase of arctic sources of cold. 



6. The Law, Rate, and Method of Flow. — The law of flow is essen- 

 tially that of rivers. 



(1.) The movement in a glacier is most rapid at or near the middle 

 line of the stream, and least so along the sides, because of friction 

 along the sides. 



(2.) The movement is most rapid at top and least so at bottom, be- 

 cause of the friction at bottom. No atmospheric friction retards the 

 movement at surface, owing to its extreme slowness. 



(3.) Where there is a bend in the stream, the movement is more 

 rapid on the convex side of the glacier than on the concave ; and the 

 medial line of greatest rapidity is nearest the convex side. 



(4.) When the stream abruptly narrows, the ice just above becomes 



