414 Mr. W. Hopkins [Jan. 31, 



but Mr. Hopkins contended that there was not the slightest evidence 

 that ice was characterized by any such ambiguous property. It might 

 bear somewhat more or less of extension or compression without dis- 

 location ; experiment had not yet determined that point with exact- 

 ness. But it might at all events be deemed certain, that ice possesses 

 no such capability of elongation as really plastic substances possessed, 

 nor such as would be sufficient to account in the least degree for that 

 elongation of the marginal parts of a glacier which must necessarily 

 result from the more rapid motion of its central portion. 



Again, to say that the capability of a glacier to move, as had just 

 been stated, was due to the breaking, bruising, and tearing of the mass, 

 was not to attribute that capability to viscosity or any other distinctive 

 property of ice. This modus operandi belonged to solid and not to 

 viscous masses ; and was, in f^-ct, a part of the process required by 

 the regelation theory, according to which the ice of a glacier must be 

 dislocated by tearing or crushing as above described, immediately 

 regaining the continuity of its mass and of its crystalline structure 

 by regelation.* In this process there was not the slightest reference 

 to any property which could, in any received meaning of the term, be 

 called viscosity. 



But, the speaker proceeded to observe, although regelation was 

 unquestionably the property of ice on which the molecular mobility of 

 a glacial mass conjoined with the preservation of its continuity de- 

 pended, it was not the physical cause of the general motion of the 

 glaciers. That cause was gravity, tending, first, to make the whole 

 mass slide bodily down the containing valley ; and, secondly, to 

 make the upper portions of the mass move faster than the lower ; 

 in consequence of the retarding action on its base, and the 

 central portions to move faster than the lateral ones, in conse- 

 quence of the retarding action of the sides of the valley. De 

 Saussure had been the first to suggest the sliding of glaciers along the 

 bottoms of their valleys ; but when a more careful attention was subse- 

 quently given to glacial phenomena by M. Agassiz, Professor Forbes, 

 and others, grave and apparently insurmountable objections were 

 urged against this view of glacier motion. It was said to be most 

 improbable, if not impossible, that a glacier should descend in this 

 manner down a surface inclined to the horizon at an angle not 

 unfrequently less than 5°, at the same time overcoming numerous 

 obstacles to its motion ; and, moreover, that if the mass did so move 

 at all, it must necessarily be with an accelerated motion, like that with 

 which a body ordinarily slides down an inclined plane, and then be 

 finally discharged from the mouth of its valley like an avalanche. 

 But whatever apparent force might belong to these objections, they 

 arose from an erroneous conception of the real nature of the mechani- 

 cal problem which the motion in question presented to us. In the 

 common case of a body sliding down an inclined plane, it was found 



* Dr. Tyndall had the kindness to elucidate this process, by repeating his ex- 

 periment on regelation before the audience. 



