46 MAN AND THE GLACIAL PERIOD. 



below. At such times the advancing water rolls onwards 

 like the surf with a perpendicular front, varying in height 

 according to the extent of the flood. 



Seasoning from these phenomena connected with mov- 

 ing water, it was naturally suggested to Professor Tyndall 

 that an analogous movement must take place in a glacier. 

 Choosing, therefore, a favourable place for observation on 

 the Mer de Glace where the ice emerged from a gorge, he 

 found a perpendicular side about one hundred and fifty 

 feet in height from bottom to top. In this face he drove 

 stakes in a perpendicular line from top to bottom. Upon 



subsequently observing 



them, Tyndall found, 



as he expected, that 



there was a differential 



motion among them as 



in the stakes upon the 



surface. The retarding 



effect of friction upon 



the bottom was evident. The stake near the top moved 



forwards about three times as fast as the one which was 



only four feet from the bottom. 



The most rapid motion (thirty-seven inches per day) 

 observed by Professor Tyndall upon the Alpine glaciers 

 occurred in midsummer. In winter the rate was only 

 about one-half as great ; but in the year 1875 the Nor- 

 wegian geologist, Helland, reported a movement of twenty 

 metres (about sixty-five feet) per day in the Jakobshavn 

 Glacier which enters Disco Bay, Greenland, about latitude 

 70°. For some time there was a disposition on the part 

 of many scientific men to doubt the correctness of Hol- 

 land's calculations. Subsequent observations have shown, 

 however, that from the comparatively insignificant glaciers 

 of the Alps they were not justified in drawing inferences 

 with respect to the motion of the vastly larger masses 

 which come down to the sea through the fiords of Green- 



er & 6 o o o f 



a. o Q o o or 



