Mr. HOPKINS, ON THE MOTION OF GLACIERS. 169 



the sliding theory, it should be shewn that the disengagement of the glacier from its bed depends on 

 the kind of weather which affects its surface and temperature." The action of the subglacial 

 currents does fully account, I conceive, for the phenomenon in question. 



(6.) It has been contended that, according to the sliding theory, the glacier ought to descend 

 with an accelerated motion. This objection never bad any real foundation, but only arose, as I 

 have shewn in my former memoir, from an erroneous conception of the nature of the retarding forces 

 which must act on the glacier during its sliding motion, whatever might be the cause of such 

 motion. My experiments, however, afford the most complete answer to the objection. 



(7.) It is said that the flow of heat from the earth is not sufficient to produce the effect which 

 this theory ascribes to it: — I reply, that all which the theory requires is, that the lower surface 

 of the glacier should be constantly kept at the temperature at which the disintegration of ice com- 

 mences. The tangential action of the bed on the bottom of the glacier will in such case be so 

 modified as to render it impossible for that action to prevent all motion. 



(8.) Another objection has been founded on the existence of glaciers of the secondary order, 

 which are observed to rest on surfaces of great inclination. Professor Forbes remarks, " M. de 

 Charpentier has very justly quoted several examples as proving, that if glaciers really slide over 

 the soil, as De Saussure supposed, these could not for a moment sustain their position at an angle 

 of 30° or more," (p. 79). M. de Charpentier, I presume, would contend that if gravity were the 

 primary cause of glacial motion, such a glacier would descend with tlie rapidity of an avalanche. 

 But it appeared from my experiments, that a mass of ice might be placed on a surface as smooth 

 as that of a paving slab at an angle of nearly 20", without descending with an accelerated motion, 

 even when the lower surface of the ice was lubricated by its being in a state of dissolution. Now 

 these secondary glaciers are generally at a great elevation, and of no great thickness, so that it 

 is highly probable that a considerable portion of their lower surfaces may be frozen to the rocks 

 on which they rest. This circumstance, together with the probable inequalities of the surface of 

 those rocks, leaves no difficulty in accounting for the want of accelerated and precipitous move- 

 ments in such glaciers as those above spoken of, nor even in those of still greater inclination. 

 They will descend down their highly-inclined beds with an unaccelerated motion, and will then 

 be precipitated, as avalanches, down the precipices which usually form their lower boundaries. 



In another part of his work, Professor Forbes appears to give an opposite phase to the 

 objection derived from secondary glaciers, and to make it rest on the assumed fact of these 

 secondary glaciers being frozen to the I'ocks throughout the whole of their lower surfaces. That 

 these glaciers are partly frozen to their beds, I have above stated to be pi'obable ; that they are 

 entirely so, no proof has been or can be offered. We possess no knowledge of them which does 

 not justify the application of the sliding theory to them, as well as to other glaciers. 



W. HOPKINS. 



Cambbidge, 

 Dec. 11, 1843. 



