14 Mr. Hopkins on the Motion of Glaciers. 



the line of greatest inclination on an inclined plane, the force 

 exerted to move along the plane depends on the height of its 

 upper above its lower extremity ; and the same is true if the 

 chain, instead of being laid in a straight line, be placed on an 

 undulating plane curve lying in a vertical plane. Similarly, 

 if the bed of a glacier be an undulating surface, the glacier 

 will descend along it with very nearly the same facility as if it 

 were plane. 



If, then, a glacier be entirely arrested in its descent, it must 

 be by local obstacles^ which exert retarding forces on the mass 

 at comparatively few points, and not, as in the case of ordi- 

 nary friction, at every point of the sliding surface. Conse- 

 quently the pressure thrown on these local obstacles might be- 

 come enormous, and much greater than glacial ice could bear 

 without being rent and broken, especially at the bottom of the 

 mass, where the texture of the ice is probably affected by its 

 temperature approximating so near to zero, and from its being 

 saturated with water at that temperature. 



It is not to be doubted that the bottom of a glacial valley 

 must be far more free from those abrupt inequalities which 

 would most interfere with the sliding of the glacier than the sides 

 of the valley, where the grinding action of the glacier must be 

 much less than on the bottom, and where there are causes 

 constantly tending, as in the sides of other valleys, to produce 

 irregularities faster in many cases than they can be smoothed 

 down by the glacier. A sudden contraction in the width of a 

 glacial valley, like that represented in the annexed diagram at 



C and C, forms probably the most serious obstacle which a 

 glacier has to encounter. In explaining how it may be sur- 

 mounted, let us suppose that the transverse portion B C C B' 

 is neither pulled onwards by the lower part C D D' C of the 

 glacier, nor pushed forward by the upper part ABB' A', so 

 that B C C B' is left to descend by its own weight along the 

 inclined plane which forms its bed. Let the inclination of 

 this plane be «; then if /3 be the least inclination of the plane 



