B. Burns — On the Mechanics of Glaciers. 301 



converted imto heat all the way down, for each impact of a molecule 

 against its fellow will be productive of a little. 



Let us now take up Mr. Goodchild's paper on the Origin of Coums 

 already referred to, and see how far Mr. Croll's theory assists him, 

 Mr. Goodchild's theory, briefly stated, is this. When a tributary glacier 

 of considerable relative size joins the main one, it thrusts the latter 

 against the opposite bank, till they together in time wear out a 

 curvilinear recess or Coum. Mr. Goodchild notices that this happens 

 with streams of water, and he insists on a perfect analogy between 

 these and glaciers. To all this I wauld have had no objection had 

 he let Mr. Croll's theory alone. It is very true that that theory 

 shows how a glacier behaves in some respects like a river and in 

 others like a solid ; but, unfortunately for Mr. Goodchild, Mr. Croll's 

 glacier is a solid when it should be a liquid, and is a liquid when it 

 should be a solid. 



To see this clearly let us first grant that the tributary glacier 

 exerts a great pressure against its principal. The main glacier is a 

 great rigid crystalline mass, with a mighty shearing force, moving 

 steadily on in its mild molecular way. Surely these characteristics 

 are not analogous to those of a river. The water of a river is free 

 to move in any direction, and no sooner is moving water joined by 

 other moving water than the two together flow off in a direction 

 and with a speed the resultant of their former directions and rates. 

 The ice of the glacier can only move by being melted, or by having 

 a pressure of 70 to 120 lbs. per square inch of shearing section 

 exerted against it. It is evident that while the tiniest stream 

 will deflect somewhat the strongest river into which it can run, the 

 tributary glacier might as well abut against a whin-dyke as the 

 main glacier; and that in this respect at least there is no analogy 

 between the glaciers and the rivers. 



Turning now to a consideration of the tributary glacier, it will 

 be seen from my remarks on crevasses that I cannot accede that the 

 tributary glacier exerts a pressure against the other, nor can I see 

 any analogy between the action of the tributary glacier and the 

 tributary river. The main glacier, as an object to be moved, is 

 a stubborn solid ; the tributary glacier, as a moving body, is slower 

 and more silent than the slowest and most silent river. The motion 

 of its molecules is that of dew, or the damp in the paper-hanging of a 

 field-geologist's bed-room, and not the rapid and continuous motion 

 of molecules of water in a river. The water of a river has momen- 

 tum, and hence its denuding power ; the ice and liquid molecules of 

 a glacier have no momentum. 



The motion of the molecules within a glacier can no more cause a 

 thrust than the rise of the sap within a tree in the spring can pull it 

 up by the roots. 



To recapitulate briefly these conclusions : Mr. Croll's theory 

 makes Mr. Goodchild's larger glacier a rigid stubborn thing, which 

 can only be moved in a lateral direction by the exercise of enormous 

 force, while it shears his smaller glacier of all its strength. To 

 accept that theory, therefore, and to ascribe the origin of Coums to 

 the confluence of glaciers is to perpetrate a blunder. 



