250 Mr. Hopkins on the Mechanism qf Glacial Motion. 



would the conviction that such would be the case involve the 

 conclusion, that the mass of earth must have that degree of 

 semifluidity wliich would cause it to descend when its lower 

 surface should adhere firmly to the rock on which it reposed ? 

 Few persons, I imagine, would draw such an inference. 



It is not my purpose, however, to dwell on general argu- 

 ments of this kind, whatever force may, in my own opinion, 

 belong to them, because, as I have already remarked, the 

 claims of the two theories will undoubtedly be determined by 

 other means. The observations required are such as shall 

 determine, as far as possible, the relative motions of the upper 

 and lower surfaces of a glacier. We may never hope to have 

 access to the bottom of a glacier in its deeper portions ; but 

 at the extremities of glaciers the amount of sliding may easily 

 be ascertained, as well as at many other points, probably, if 

 sought for, along their flanks ; fissures, also, of considerable 

 depth are not unfrequently met with, in which the deviation 

 from verticality, if it exist, might be easily determined ; and 

 though the evidence thus obtained might not afford positive 

 demonstration with reference to the deepest portions of a 

 glacier, still, should it all concur in showing an approximate 

 equality in the motions of the upper and lower surfaces, every 

 candid and impartial mind must admit, I conceive, the sliding 

 in preference to the viscous theory ; but if, on the contrary, it 

 should be proved that the velocity of the upper bears a large 

 ratio to that of the lower surface, the claims of the latter 

 theory must be at once admitted. 



With respect to the mechanical investigations involved in 

 the foregoing discussion, the case is widely different. They 

 are not dependent on future observations, because they are 

 based on the already well-established fact of the excess of the 

 central motion over that of the sides of the glacier, and will be 

 equally applicable whichever of the two theories may be 

 adopted. In the development of my own views, I have en- 

 deavoured to be as explicit as possible, to prevent misappre- 

 hension ; and for the same reason I have endeavoured to be 

 equally explicit in pointing out what I conceive to be the 

 errors in Prof. Forbes's views respecting the mechanism of 

 the motion in question. This desire to be explicit may pos- 

 sibly have given an appearance of severity to some of my cri- 

 ticisms, in having, perhaps, led me to attach more definite 

 tentive subsoil, on the brow of a hill, descended very slowly down the side 

 of the hill at the inclination of about 10°. It appeared to be of the extent 

 of several acres, and was strongly fissured transversely, in a manner exactly 

 analogous to a glacier, except that the crevasses were not curved, but had 

 their mean directions more directly transverse, owing to there being no 

 excess of the central over the lateral velocity. 



