156 VISCOUS THEORY OF GLACIER MOTION. [1846. 



therefore endowed with very feebly hydrostatic qualities.* This is 

 demonstrated on the one hand by the extreme smallness of their 

 motions, and on the other by the insignificant streams of water 

 to which they give birth even in the height of summer. ' In any 

 individual glacier the velocity of the parts must (on any theory) 

 vary with the area of section through which the ice stream has 

 to pass ; but yet it may happen that the contraction of a valley, 

 if not accompanied (as is often the case) with an increased slope, 

 will oppose so great a resistance to the efflux of the mass, that 

 under intense longitudinal compression its forward motion is re- 

 tarded, and the condition of uniform discharge is satisfied by the 

 accumulation of the ice in a vertical direction, the rise of the 

 surface being necessarily accompanied with a thrust from below 

 upwards, and a sliding of the particles over one another in that 

 direction. This appears conclusively to be the case for a great 

 extent of the lower part of the glacier of the Aar, as already 

 mentioned, and affords the most direct evidence which could be 

 desired, that the kind of internal motion necessary for producing 

 the frontal dip in the veined structure (which arises from tear- 

 ing or crushing in sliding in the vertical plane)f was correctly 

 foreseen. 



The law of velocities at different points of the axis of a 

 glacier from its origin towards its termination, must evidently 

 depend upon the configuration of each particular glacier. It 

 may be constantly increasing from the origin to the extremity, 

 it may be diminishing, or it may have alternations of increase 

 and diminution ; and upon this circumstance the frequency and 

 magnitude of the crevasses will mainly depend. But the regime 



* [This consideration (or rather the observations on which it is founded) proves 

 convincingly the important part which water plays as a component part of a glacier. 

 A bed of mere indurated snow or neve, though it is evidently more incoherent than 

 glacial ice, owing to its dryness, is, as we see, far less mobile than the glacier pro- 

 per. It acts like sand, transmitting pressure with difficulty (see page 142) ; it is 

 also, notwithstanding its state of division into granules, intrinsically harder than 

 ice perfectly lubricated with water, and probably very slightly colder because dry. 

 See below, page 166, and later in this volume, the paper On some Properties of Ice 

 near its melting point. Nov. 1858.] 



f Seventh Letter on Glaciers. [Page 59, above.) 



