106 SCIENCE OF COMMON THINGS. 



What is a glacier f Icebergs. Line of perpetual snow. 



600 What is a glacier f 



The glacier only exists upon mountains whose sum- 

 mits are covered with perpetual snow. The snow upon 

 the higher parts becomes somewhat softened during 

 the summer, and in the winter is again hardened nearly 

 to ice. In the succeeding summer, the action of the 

 sun, and the internal heat of the earth, detach large 

 masses loaded with recently deposited snow into the 

 neighboring valleys, where, being accumulated, and 

 the crevices filled with snow or water which at Jast 

 hardens to ice, they form huge seas of ice, or a glacier / 

 in French, mers-de-glace. 



667 Do the glaciers continue to increase year by year f 



Very many of them do ; and in Switzerland many 

 valleys, once fertile, are now filled with glaciers. From 

 the bottom of the glacier streams of water constantly 

 issue, and it is from such sources that the rivers Rhine 

 and Rhone of Europe take their rise. 



668 How are the gigantic icebergs formed which are found floating at 

 some seasons in the Atlantic ? 



They are portions of great glaciers formed in the 

 northern regions, which become detached and float in 

 the sea. 



669 How high are icebergs sometimes seen f 



Sometimes exceeding 300 feet in height. 



670 At what elevation above the surface of the earth, at the equator, 

 will water remain frozen ? 



At an elevation of about 15,000 jjfrf& 



671 At what elevation in the straits of Magellan will water remain 

 frozen f 



At about 4000 feet. 



673 What is the point where water remains frozen called t 



The line of perpetual snow. 



673 Why are not all places which lie under the same parallel of lati- 

 tude of the same temperature ? 



Because various disturbing circumstances tend to vary 

 the mean temperature. 



