CLOUDS AND RIVEES, ICE AND GLACIERS. 77 



cross the centre. But at the Montanvert it would again 

 lie east of the centre ; hence between the Ponts and the 

 Montanvert the centre must be crossed a second time. 

 If there were further sinuosities upon the Mer de Glace 

 there would be further crossings of the axis of the glacier. 



187. The points on the axis which mark the transition 

 from eastern to western bending, and the reverse, may 

 be called points of contrary flexure. 



188. Now what is true of the Mer de Glace is true of 

 all other glaciers moving through sinuous valleys ; so 

 that the facts established in the Mer de Glace may be 

 expanded into the following general law of glacier mo- 

 tion : — 



When a glacier moves through a sinuous valley, the locus 

 of the point of maximum motion does not coincide with 

 the centre of the glacier, but, on the contrary, always 

 lies on the convex side of the central line. The locus is 

 therefore a curved line more deeply sinuous than the valley 

 itself, and crosses the axis of the glacier at each point of 

 contrary flexure. 



189. The dotted line on the Outline Plan (page 68) 

 represents the locus of the point of maximum motion, 

 the firm line marking the centre of the glacier. 



190. Substituting the word river for glacier, this law 

 is also true. The motion of the water is ruled by pre- 

 cisely the same conditions as the motion of the ice. 



191. Let us now apply our law to the explanation 

 of a difficulty. Turning to the careful measurements 



