GLACIAL MORTON. 



45 



For example, the combined width of the three tributaries 

 uniting to form the Mer de Glace is 2,597 yards ; but a 

 short distance below the junction of these tributaries the 

 total width of the Mer de Glace itself is only 893 yards, 

 or one-third that of the tributaries combined. Yet, though 

 the depth of the ice is probably here much greater than in 

 the tributaries, the rapidity of movement is between- two 

 and three times as great as that of any one of the branches.* 



From Tyndall's observations it appears also that the 

 line of most rapid motion is not exactly in the middle of 

 the channel, but is pushed by its own momentum from 

 one side to the other of the middle, so 

 as always to be nearer the concave side ; 

 in this respect conforming, as far as its 

 nature will permit, to the motion of 

 water in a tortuous channel. 



It is easy to account for this differen- 

 tial motion upon the surface of a glacier, 

 since it is clear that the friction of the 

 sides of the channel must retard the mo- 

 tion of ice as it does that of water. It 

 is clear also that the friction of the bot- 

 tom must retard the motion of ice even 

 more than it is known to do in the case 

 of water. In the formation of breakers, 

 when the waves roll in upon a shallow- 

 ing beach, every one is familiar with the 

 effect of the bottom upon the moving mass. Here friction 

 retards the lower strata of water, and the upper strata 

 slide over the lower, and, where the water is of sufficient 

 depth and the motion is sufficiently great, the crest breaks 

 down in foam before the ever-advancing tide. A similar 

 phenomenon occurs when dams give way and reservoirs 

 suddenly pour their contents into the restricted channels 



Fig. 17. 



See Tyndall's Forms of Water, pp. 78-82. 



