160 ELEMENTS OF SCIENCE 



scoop out large excavations, so deepening the depressions 

 in the mountain sides and in the valleys which they 

 traverse. The length of Swiss glaciers is sometimes 

 twenty miles, their breadth occasionally two or three 

 miles, and their depth 600 feet. They melt slowly as 

 they descend, the water flowing in tunnels beneath them, 

 and issuing from under ice arches at their lower extremity. 

 Masses of rocks, or boulders, and many stones are carried 

 along by them, accumulating at the glacier's lower ter- 

 mination, such accumulations being known as moraines. 



In high latitudes, great masses of glacier will break off 

 into the sea and float away to warmer climes, as icebergs, 

 carrying with them large masses of rock and boulders 

 with a large quantity of stones and mud. They have 

 been seen so large as to be many miles in circumference 

 and 300 feet high. Such a mass must be vast indeed, 

 seeing that for every cubic foot above the sea's surface 

 there must be eight cubic feet below it. 



But the condensation which appears occasionally as 

 snow, but generally as rain, takes place very unequally 

 over the earth's surface. The tropics are most abundantly 

 watered, parts of Brazil receiving annually as much 

 as 270 inches of rain, and Cherra Poonjee, in Assam, 

 500 inches. On the east of the Andes, however, 

 there is a narrow tract of land which is rainless because 

 the constant western winds are drained of their water 

 as they pass over the snow-capped mountains. That 

 great desert, the Sahara, of Northern Africa, is rainless, 

 because the moist winds and clouds it may receive from 

 the Mediterranean find in it not a condenser but, on 

 aecount of its heat, a vapouriser, which, as before said,* 

 makes clouds vanish. The great tableland of Gobi, in 



* See ante, p. 158. 



