370 



MOTION OF GLACIERS. 



[Ch. XVI. 



enormous 



as 



a glacier on its bed is so great, that the vertical direction 

 wonld always be that of least resistance, and if a considerable 



mass 



freezing, it would tend to increase its thickness, rather than 



ird progress. He also contended (and 

 illustrated by many ingenious experi- 



downw 



ments) that a glacier can move along an extremely slight 

 slope, solely by the influence of gravitation, owing to the 

 constant dissolution of ice in contact with the rocky bottom, 

 and the number of separate fragments into which the glacier 

 is divided by fissures, so that freedom of motion is imparted 

 to its several parts somewhat resembling that of an imperfect 

 fluid. To this view Principal James D. Forbes objected, 

 that gravitation would not supply an adequate cause for the 

 sliding of solid ice down slopes having an inclination of no 

 more than four or five degrees, still less would it explain how 

 the glacier advances where the channel expands and con- 



tracts. 



Chamouni 



being 2,000 yards wide, passes through a strait only 900 yards 

 in width. Such a gorge, it is contended, would be choked 



even if it be broken 

 The same acute observer 



up by the advance 



mass 



up 



into numerous fragments. 



remarked, that water in the fissures and pores of glaciers 

 cannot, and does not part with its latent heat, so as to freeze 

 every night to a great depth, or far in the interior of the 



mass 



Had 



of the glacier would have occurred about sunset, when the 



must 



been at first assumed by those who favoured that hypothesis, 

 that the mass moved faster at the sides, where the melting of 



from 



precipices 



Agassiz appears to have been the first to commence, 



m 



1841, aided by a skilful engineer, M. Escher von der Linth, 



a series 



of exact measurements to ascertain the laws oi 



glacier motion, and he 



soon 



discovered, contrary to his 

 preconceived notions, that the stream of ice moved more 

 slowly at the sides than at the centre, and faster in the 



