48 AQUEOUS AGENCIES. 



The only difference is the degree of ramification. Streams ramify 

 almost infinitely, while glaciers seldom have more than three or four 

 tributaries. Fig. 37 is a map of the Mont Blanc glacier region. By 

 inspection of this map it will be seen that the Mer de Glace, m, receives 

 four tributaries, marked /, I, g, etc. On page 55 is an enlarged view 

 of the same glacier with its tributaries. 



Motion of Glaciers. — Again, we have said in our definition that gla- 

 ciers are in constant motion. By the law of circulation, constant down- 

 ward motion is as necessary to the idea of a glacier as it is to that of a 

 river, since both the glacier and the river carry away the excess of sup- 

 ply over evaporation. But a glacier, though in constant motion, never 

 passes beyond a certain point, where the slow downward motion is 

 exactly balanced by the melting of the ice by sun and air. This point 

 is called the lower limit of the glacier. As long as conditions remain 

 unchanged, the lower end of the glacier remains exactly at the same 

 point, although the substance of the glacier moves always downward. 

 But if external conditions change, the point of the glacier may move 

 upward or downward. There are two opposing conditions which de- 

 termine the position of the point of the glacier, viz., the rate of mo- 

 tion and the rate of melting. Thus, during a succession of cool, damp 

 years, the melting being less rapid, the point of the glacier moves 

 slowly down, sometimes invading cultivated fields and overturning 

 huts, until it finds a new point of equilibrium. During a succession of 

 warm and dry years, on the contrary, the melting being more rapid, 

 the point retreats, to find a new point of equilibrium higher up the 

 mountain. Again, during a cycle of years of heavy snow-fall, the gla- 

 cier is flooded, its motion increased, and its point advances ; while 

 during a cycle of smaller snow-fall its dimensions shrink, its motion 

 is retarded, and its point retreats. But, whether the point be station- 

 ary, or advance or recede, the substance of the glacier is ever moving 

 steadily onward. It may be compared to those rivers, in dry, sandy 

 countries, which run ever toward the sea, but never reach beyond a 

 certain point, being absorbed by the sand. 



Graphic Illustration. — These facts may be graphically represented 

 as follows : Taking first the motion constant in time, and the melting 

 variable, let a d (Fig. 38) equal the length of the mountain-slope, 

 and the line a o ( = c d) the velocity of the glacier-motion taken as 

 uniform. This velocity varies with the slope, as will be seen hereafter, 

 but is little affected by the elevation. It may be taken, therefore, as 

 the same in every part of the slope, and therefore correctly represented 

 by equal lines, i. e., by the ordinates of the parallelogram abed. The 

 melting power of the sun and air, on the contrary, regularly increases 

 from the top, where it is almost nothing, to the bottom of the mount- 

 ain. We will, therefore, represent it by the increasing ordinates of the 



