AQUEOUS AGENCIES. 53 



tributaries, only the tributaries of glaciers are far less 

 numerous than those of rivers. 



We have said that glaciers are in continual, slow motion 

 down the valley ; yet in temperate climates they do not 

 reach the sea they do not reach beyond a certain point, 

 called the lower limit of glaciers. Under constant condi- 

 tions the snout of the glacier remains unmoved at this 

 place, even though the glacier is in constant current- 

 motion. This apparent anomaly may be explained thus : 

 The glacier may be regarded as under the influence of 

 two opposite forces. Gravity urges it by slow motion 

 downward, and, if this acted alone, the glacier would 

 run into the sea. But the ice is constantly melted, more 

 and more, as the glacier presses downward into warmer 

 regions ; if this alone acted, the point of the glacier 

 would retreat to the summit-snow. Now, where these 

 two forces one tending to lengthen, the other to shorten 

 the glacier balance each other, is found the lower limit 

 of the glacier where the snout rests unmoved. Some- 

 times, after a succession of cool, moist years, or a succes- 

 sion of heavy snowfall years, the melting is less rapid or 

 the motion more rapid, and the snout of the glacier will 

 slowly advance, perhaps invading cultivated fields and 

 overturning houses. Sometimes, on the contrary, from 

 more rapid melting or less rapid motion, the snout will 

 recede, strewing debris in its former bed. But, whether 

 the snout stands still, or moves forward, or moves back- 

 ward, the matter of the glacier is moving constantly 

 downward. In this respect, glaciers are like rivers in 

 certain dry regions. These rivers rise in the mountains, 

 run a certain distance, but never reach the sea never 

 pass a certain point where the supply is balanced by 

 waste from evaporation. 



We have said that glaciers reach far below the line of 

 perpetual snow. In the Alps, for example, the lower 

 limit of glaciers is 5,000 feet below the snow-line. This 



