110 ICE AND GLACIEKS. 



valleys, who have the glaciers constantly before their eyes, often 

 -cross them, and in so doing make use of the larger blocks 

 of stone as sign posts detect this motion by the fact that their 

 guide posts gradually descend in the course of each year. And 

 as the yearly displacement of the lower half of the Mer de 

 Glace at Chamouni amounts to no less than from 400 to 600 

 feet, you can readily conceive that such displacements must 

 ultimately be observed, notwithstanding the slow rate at which 

 they take place, and in spite of the chaotic confusion of crevasses 

 and rocks which the glacier exhibits. 



Besides rocks and stones, other objects which have acci- 

 dentally alighted upon the glacier are dragged along. In 1788 

 the celebrated Genevese Saussure, together with his son and a 

 company of guides and porters, spent sixeen days on the Col du 

 Geant. On descending the rocks at the side of the cascade of the 

 Glacier du Geant, they left behind them a wooden ladder. This 

 was at the foot of the Aiguille Noire, where the fourth band of 

 the Mer de Glace begins ; this line thus marks at the same time 

 the direction in which ice travels from this point. In the year 

 1832, that is, forty-four years after, fragments of this ladder were 

 found by Forbes and other travellers not far below the junction 

 of the three glaciers of the Mer de Glace, in the same line (at 

 s, Fig. 19), from which it results that these parts of the glacier 

 must on the average have each year descended 375 feet. 



In the year 1827 Hugi had built a hut on the central 

 moraine of the Unteraar Glacier for the purpose of making 

 observations ; the exact position of this hut was determined by 

 himself and afterwards by Agassiz, and they found that each 

 year it had moved downwards. Fourteen years later, in the 

 year 1841, it was 4,884 feet lower, so that every year it had on 

 the average moved through 349 feet. Agassiz afterwards found 

 that his own hut, which he had erected on the same glacier, had 

 moved to a somewhat smaller extent. For these observations a 

 long time was necessary. But if the motion of the glacier be 

 observed by means of accurate measuring instruments, such as 

 theodolites, it is not necessary to wait for years to observe that ice 

 moves a single day is sufficient. 



