1 2 Mr. Hopkins on the Motion of Glaciers. 



ting the ice by the manner in which they cut out for them- 

 selves their own channels, thus assisting greatly in the degra- 

 dation of the surface. Its effect on the lower surface of the 

 glacier is probably greater than on the upper, on account of 

 the hydrostatic pressure under which it must there act. The 

 descending water must reach the bed of the glacier at almost 

 every point of it, and cannot afterwards collect and proceed in 

 uninterrupted channels, because if such channels were once 

 formed, they must necessarily be immediately destroyed, or 

 at least impeded at numerous points by the motion of the gla- 

 cier. The existence of such impediments to the motion of the 

 water, and the consequent formation of subglacial reservoirs, 

 is proved by the continued flow of the streams which issue 

 from thelower extremities of glaciers during the night, though 

 the supply from the upper surface is entirely stopped imme- 

 diately after sunset, when the melting ceases, and does not re- 

 commence till a considerable time after sunrise the next morn- 

 ing. During the intervening ten or twelve hours the whole 

 of the water beneath the glacier at sunset would necessarily 

 discharge itself if its course were unimpeded, even from the 

 longest and least inclined of the alpine glaciers, before sunrise 

 the next morning; whereas the volume of water issuing from 

 the glacier of the Aar is very little less in the morning than in 

 the evening. This equable supply can only arise from the 

 discharge during the night from reservoirs formed during the 

 day. Hence it will follow, that these subglacial currents, com- 

 mencing from almost every point of the glacier, will be forced 

 under every part of it by hydrostatic pressure, by which, as 

 above asserted, its disintegrating action on the lower surface 

 of the ice will doubtless be increased. 



M. Agassiz appears to entertain the opinion, that glaciers, 

 throughout the greater part of their length, are firmly frozen 

 to their bed, and, consequently, that all sliding, except at their 

 lower extremities, is out of the question ; and others, I believe, 

 have also held the same opinion. The preceding investigation, 

 however, leaves no doubt, I conceive, that such cannot possibly 

 be the state of the inferior surfaces of glaciers of considerable 

 thickness. 



7. M. de Charpentier has insisted on the small inclinations 

 of the beds of many glaciers as a conclusive objection against 

 the sliding theory, and Prof. Forbes has spoken of the " in- 

 tense friction" exerted by the bed of a glacier*, and of the 

 impossibility of its sliding uniformly, if it should slide at all. 

 These objections have arisen from an imperfect conception of 

 the real action of the bed of a glacier on the ice in contact 

 with it in a state of disintegration, assuming the general sur- 

 * See note *, p. 3. 



