\ 



Mr. HOPKINS, ON THE MOTION OF GLACIERS. 57 



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 tlie motion of the glacier. 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 the lower extremities of glaciers during the night, though the supply 

 from the upper surface is entirely stopped immediately after sunset, when the melting ceases, 

 and does not recommence till a considerable time after sunrise the next morning. 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, commencing 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. 



SECTION III. 

 Phenomena depending upon the 3Iotio7i of Glaciers. 



6. Relative Velocities of the Central and Lateral Portions of a Glacier. — The central 

 part of a glacier moves considerably faster than its sides, but, according to Professor Forbes*, the 

 change of velocity takes place not far from the lateral boundaries, the whole central portion 

 moving with nearly the same velocity. In the month of August last summer, the central part 

 of the Aar glacier, near the cabane of M. Agassiz, was moving at about the rate of a foot a day, 

 while near the sides it was less by one third or one half. On the Mer de Glace the motion 

 appears to be generally greater, in the ratio of about 3 : 2, but varying in different parts of the 

 glacierf. The difference between the central and lateral motions seems to be less than in the 

 former case. 



On the Mer de Glace the velocity near the lower extremity appears to be somewhat greater 

 than near the upper one. On other glaciers no adequate observations on this point have yet been 

 made. 



7. Crevasses or Fissures — The fissures which traverse a glacier are among its most distinct 

 and striking phenomena. When the glacial valley contracts in descending, the following facts 

 appear to be established. 



The fissures are transverse and curved, having their convexity turned towards the upper 

 extremity of the glacier. 



Systems of fissures, preserving a certain identity of character with respect to number and 

 form, remain fixed in position, not with reference to the moving mass, but with reference to the 

 fixed objects around. It is not however to be understood that each fissure of a system remains 

 absolutely stationary, but that each system remains so in the same sense in which what may be 

 termed a system of breakers on the sea-shore may be said to be stationary, although every suc- 

 cessive wave- is in constant motion. In like manner every fissure must move through a certain 

 space with the glacier, and then disappear by closing, or be so modified as to lose its identity J; 



" I^ctlcrit to Processor Jameson — Ktlinburgh Journal of 

 Scienee. 

 t Ibid. 



t I consider a fissure to remain identically the same so long 

 aa it continues to intervene between the same identical portions 

 of ice. 



Vol.. vni. I'AitT I. H 



