MOVEMENT OF GLACIERS 35 



incredible moving thing which no one would ever have 

 foreseen as the product of a heap of snow, no matter how 

 vast or where accumulated. As it moves downwards the 

 mass is subjected to immense pressure, both from its own 

 weight resting on the rocks, and from lateral pressure or 

 squeezing from the sides of the bed or hollow, which hold 

 it as a river of water is held by its banks. The con- 

 tinuous and varied pressure and pull, tear, and squeeze of 

 the huge mass, in its irregular bed, alter a great deal the 

 character of the ice as it advances. 



Glaciers differ in length according to the amount of 

 snow which is annually furnished by the high collecting 

 ground of the nev6, and also according to the steepness of 

 the bed along which they travel, as well as to some extent 

 in relation to the greater or less heat of the valleys into 

 which they descend. The fact that the ice which is melting 

 away at the snout, or lower end, of a glacier has gradually 

 descended from above, has been long known by the 

 mountain folk, but it was only in the last century that 

 the rate of descent was measured. It varies from 150 ft. 

 to 1 000 ft. a year ; it varies in different parts of the same 

 glacier, and at different seasons of the year, and in different 

 years. In the summer an average sample of a glacier 

 will advance a foot and a half a day in the middle, and a 

 foot or less at the sides. It has been calculated that a 

 particle of ice would take about 500 years to descend from 

 the summit of the most beautiful of all the great Swiss 

 mountains — the Jungfrau — to the end of her greatest 

 glacier, that called the great Aletsch, which expands its 

 melting " snout " below the Bel Alp over the Rhone valley. 

 The Swiss glaciers had been, on the whole, increas- 

 ing in size for some 500 years until 1820, when they 

 retreated until 1840, and again advanced until i860. 

 Since then they have greatly diminished, though some are 

 now advancing again. Many are the lamentations of old 



