TJie Theory of Glacier motion. 121 



the pressure to which they are respectively subjected. The 

 friction occasioned by the nature of the channel or 

 surface over which they move, and the viscosity^ or mutual 

 adhesiveness, of the particles of the semi-fluid, which 

 prevents each from taking its own course, but subjects 



a.11 to a mutual restraint The quantity of viscidity or 



imperfect mobility in the particles of fluids may have every 

 •conceivable variation ; the extremes are perfect fluidity on 

 the one hand, and perfect rigidity on the other. A good 

 example is seen in the process of consolidation of common 

 plaster of Paris, which, from a consistency not thicker than 

 that of milk, gradually assumes the solid state, through 

 every possible intermediate graduation. Even water is not 

 completely mobile ; it does not run through capillary tubes, 

 and a certain inclination or fall is necessary to make it flow. 

 Water will run freely on a slope of 6 inches in a mile, or a 

 fall of one io,oooth part ; another fluid might require a fall 

 of I in 1,000 ; whilst many bodies may be heaped up to an 

 angle of several degrees before their parts begin to slide over 

 one another. 



" Thus a substance apparently solid may, under great 

 pressure, begin to yield ; yet that yielding or sliding of the 

 parts over one another may be quite imperceptible upon 

 the small scale, or under any but enormous pressure. A 

 column of the body itself is the source of the pressure of 

 which we have now to speak. 



"Even if the ice of glaciers were admitted to be of a 

 nature perfectly inflexible so far as we can make any 

 attempt to bend it by artificial force, it would not at all 

 follow that such ice is rigid, when it is acted on by a column 

 of its own material, several hundred feet in height. Pure 

 fluid pressure, or what is commonly called hydrostatical 

 pressure, depends not at all for its energy upon the slope of 

 the fluid, but merely upon the difference of level of the two 

 connected parts or ends of the mass under consideration. 



