Royal Society. 301 



Glaciers are very generally hemmed in by precipitous rocks which 

 determine their contour or ground plan ; they have often to make 

 their way through contracted gorges where the ice occupies (as in 

 the case of the Aler de Glace of Chamouni), within a short distance, 

 a channel but half as wide as it did before. Yet the glacier, pre- 

 serving its continuity as a whole, expands or contracts in conformity 

 with the irregularities, not only of its lateral walls, but of its bed, 

 forcing itself over obstacles, or even occasionally allowing itself to 

 be cleft into two branches by them, and closing again into a united 

 mass after the insular obstruction has been past. To speak of such 

 resistances of the channel to the progress of the ice as mere friction, 

 or of a glacier considered as a solid body and in its whole extent 

 (or in any considerable part of it) as having an angle of repose, as in 

 the case of a substance with a flat base resting on an inclined plane, 

 is evidently inadmissible and tends to mislead. The valley of the 

 Mer de Glace might have almost any possible inclination before the 

 ice would tend to slide out of it en Jiiasse, for it is moulded to every 

 sinuosity or protuberance of the bed, whether vertical or horizontal. 

 Let Mr. Moseley imagine a sheet of lead having the ground plan of 

 the Mer de Glace and confined by margins of wood accurately 

 adapted to it, and he will see that unless lead were so ductile as to 

 be entitled to the appellation of a semifluid, no motion could possibly 

 result, however great might be the slope on which it lay. 



I am sorry to find that Mr. Moseley denies entirely (p. 341) the 

 viscous or plastic structure of a glacier as " not consistent with the 

 fact that no viscosity can be traced in its parts when separated." 

 The answer to this objection seems to be merely this ; that the 

 viscosity, though it cannot be " traced" in the parts, if very minute, 

 nevertheless exists there, as unequivocally proved by experiment on 

 the large scale, or even on spaces several yjirds or fathoms in extent*. 

 The plastic condition of a glacier is, as I have repeatedly stated, no 

 longer an hypothesis, but a. fact, since I have in many places demon- 

 strated that, account for it as we may, different portions of the same 

 continuous mass of ice are moving at the same moment with dif- 

 ferent velocities. That a small piece of ice is not sensibly plastic, is 

 not more strange than that the tine blue colour so perceptible in the 

 glacier totally vanishes in its constituent fragments. That ductility 

 and fragility are not incompatible qualities, is shown by the fact, that 

 sealing-wax at moderate atmospheric temperatures will mould itself 

 {with time) to the most delicate inequalities of the surface on which 

 it rests, under a pressure of not more than half an inch of its sub- 

 stance, but may at the same time be shivered to atoms by a blow 

 with a hammer. 



The question of plasticity, however, affects only mediately Mr. 

 Moseley's theory of the primary cause of motion by dilatation and 

 contraction. According to the views I support, the dilatation and 

 contraction of the ice of glaciers (assuming it to exist) would be in- 

 efficient to move the mass unless it moved plastically ; and if it 

 moves plastically, the supposition of its thermal expansion is, at all 

 * See Phil. Trans. 1846, p. 162, and Phil. Magazine (1845), xxvi. p. 414. 



