MOTION OF GLACIERS AND ITS LAWS. 59 



Motion of Glaciers and its Laws. 



Evidences of Motion. — That glaciers move slowly down their valleys 

 was long known to Alpine hunters. Kude experiments of the first scien- 

 tific explorers confirmed this popular notion. Hugi in 1827 built a hut 

 upon the Aar glacier. This hut was visited from year to year by scien- 

 tific explorers and its change of position measured. In 1841 Agassiz 

 found that it had moved 1,428 metres in fourteen years, or about 100 

 metres (330 feet) per annum. The ruins of Agassiz's hut (Hotel 

 Neuchatalois), built in 1840, were found in 1884. They had moved in 

 forty-four years 7,900 feet.* Numerous other observations from year to 

 year by Agassiz and others, on the position of conspicuous bowlders ly- 

 ing on the surface of glaciers, confirmed these results and placed the fact 

 of glacier-motion beyond doubt. But the most important observations 

 determining both the rate and the laivs of glacier-motion were made 

 in 1842 by Prof. Agassiz on the Aar glacier, and Prof. Forbes on the 

 Mer de Glace. By these experiments, carefully made by driving stakes 

 into the glacier, in a straight row from one side to the other, and ob- 

 serving the change in the relative position of the stakes, it was deter- 

 mined that the center of the glacier moved faster than the margins. 

 This differential motion is the capital discovery in relation to the mo- 

 tion of glaciers. It is claimed by both Agassiz and Forbes. It had, 

 however, been previously distinctly stated, though not proved, by Bishop 

 Rendu. 



Laws of Glacier-Motion. — The term differential motion is a con- 

 densed expression for all the laws of glacier-motion. It asserts that 

 the different parts of a glacier do not move together as a solid, but 

 move among themselves in the manner of a fluid. A glacier moves 

 like a fluid, though a very stiff, viscous fluid ; its motion may therefore 

 be rightly called viscoid. We will mention some 

 of the most important laws of fluid motion, and 

 show that glaciers conform to them : 



1. TJie Velocity of the Central Parts is greater 

 than that of the Margins. — This well-known law 

 of currents, the result of friction of the fluid 

 against the containing banks, was completely proved 

 in the case of glaciers by the experiments of Agas- 

 siz and Forbes, and recently confirmed in the most 

 perfect manner by Tyndall. A line of stakes, ' Fl6- 49> 



a h c d ef g, placed in a straight row across a gla- 

 cier, becomes every day more and more curved, as seen in Fig. 49. 

 The exact rate of motion for each stake is easily measured by the 



* Nature, vol. xxx, p. 477, 1884. 



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