Mr. Hopkins on the Mechanism of Glacial Motion. 249 



that such is the case. The only reason which has been 

 assigned in favour of that opinion is derived from the ana- 

 logy which such motion would bear to the motion of the 

 centre of a glacier relatively to its sides. It is said that as 

 the centre moves faster than the sides, so may we conclude 

 that the upper portion must move faster than the lower. Now 

 it is not denied that the argument is good to a certain extent, 

 but the question is entirely one of degree. We have no right 

 to conclude that the relative motion in the one case will be 

 equal to that in the other, since the conditions in the two 

 cases are different. The depth of a glacier is much less than 

 its semi-width ; the continuity of the mass is much broken by 

 vertical planes, and the relative motion of the centre and sides 

 is thus facilitated, whereas the mass is probably altogether 

 unbroken by horizontal planes; also the vertical pressure, 

 where the depth is considerable, is probably much greater 

 near the bottom than the horizontal pressure near the sides of 

 the glacier. All these are reasons why the relative motion of 

 the upper and lower surfaces should be less than that of the 

 centre and sides of the glacier. If, however, we allow these 

 relative motions to be equal, it will not suffice the require- 

 ment of the theory ; for it would still only account for about 

 half the observed motion, since the excess of the velocity of 

 the centre over that of the sides does not probably exceed 

 half the real motion of the centre of the glacier. It would 

 therefore be necessary generally that the excess of the motion 

 of the upper surface over that of the lower one, should be 

 nearly twice as great as that of the central over the lateral 

 motion. Without asserting that this conclusion involves a 

 mechanical impossibility, it appears to me extremely difficult 

 to admit any theory in which it stands as the immediate con- 

 sequence of a fundamental assumption unsupported by direct 

 evidence. 



To illustrate the inconclusiveness of this reasoning from 

 analogy, let us conceive the ice which constitutes one of the 

 glaciers of Chamouni or Grindelwald, in which the inclination 

 of the bed is probably 10° or upwards, to be removed, and its 

 place supplied by an equal quantity of common earth ; and 

 let us suppose the same quantity of water as that which now 

 runs beneath the glacier, to make its way between the mass of 

 earth and its bed. What engineer would recommend the 

 erection of a house on such a foundation ? I have no doubt 

 but that we should have the phaenomenon of what might be 

 termed a mud glacier in slow but constant motion*. But 



* I have, in fact, witnessed a phaBnomenon precisely of this kind. The 

 upper stratum of earth, to the depth of four or five feet, resting on a re- 

 Phil. Ma2. S. 3. Vol. 26. No. 172. March 1845. S 



