The Theory of Glacier motion. 119 



horizontal cohesion, as when it is divided into pyramidal 

 masses. This of course destroys any determinate inequality 

 of motion, each mass moving singly {Edin. New Phil. Journ.^ 

 •October, 1842). 



In his famous fourth letter, published in the Edin. New 

 Phil Journ.y January, 1848, Forbes urges most emphatically 

 that ice moves like a plastic body, and after quoting a number 

 of facts in regard to the change in crevasses, inequalities, &c., 

 he says, " all these facts, attested by long and invariable 

 experience, prove that the ice of glaciers is insensibly 

 and continually moulding itself under the influence of 

 external influences, of which the principal, be it remarked, 

 is its own weight affecting its figure, in connection with the 

 surfaces over which it passes, and between which it struggles 

 onwards. It is in this respect, absolutely comparable to the 

 water of a river, which has here its deep pools, here its 

 constant eddy, continually changing in substance, yet ever 

 the same in form " {id. 34, 4). " The centre of the glacier 

 stream," he again says, "is urged onward by pressure from 

 above, which is there resisted less than at the sides and bottom, 

 owing to the comparative absence of friction. The lateral 

 parts are dragged onwards by the motion of the centre, 

 and move also, but it is quite compatible with this 

 idea of semi-fluid motion that the bottom of the glacier 

 should remain frozen to its bed, as some writers have 

 supposed to be the case, though I am far from asserting 



this to be the fact, or even supposing it probable." 



"The motion of a glacier resembles that of a viscid 

 fluid, not being uniform in all parts of the transverse 

 section, but the motion of the parts in contact with the 

 walls being determined, mainly, by the motion of the 

 centre." And he concludes this famous letter by urging 

 that the admission of some fluid motion in a glacier 

 seems to explain the chief facts of glacier movement : — - 

 (i) That it is more rapid at the centre than at the 



