298 D. Burns — On the Mechanics of Glaciers. 



bottom of the glacier melts and moves a bit downwards. It tben 

 freezes, and in doing so gives out the heat which melted it. The 

 heat thus liberated now melts the next molecule in a line up the 

 length of the glacier from A, and this molecule, in the form of 

 water, and denominated B, trickles down to A and freezes. The 

 heat is again liberated, and again it melts a molecule up-hill, which 

 behaves as did the others ; and so on for any distance. In this wa}'' 

 a whole line of molecules move a short distance down-hill, and bj'- a 

 repetition of such movements among all its molecules does a glacier 

 make progress. 



Now it seems strange that molecule A should, on freezing, give 

 its heat to B, from which it is some distance removed, and should 

 impart none to the molecule on which it rests, or to those on either 

 side. This is difficulty number one. But, supposing the molecules 

 to be perfectly accommodating in this respect, their downward pro- 

 gress is only helped by heat passing along the glacier from its lower 

 end upwards. Let us take the molecules A, B and C somewhere 

 within or on the surface of the glacier. Supposing now B is melted 

 by the rays of the sun. When a solid, it was in contact with both A 

 and C, and therefore on melting it cannot move. True, when 

 liquid, it is reduced about one-tenth in size, and in consequence its 

 centre may move a small fraction of its diameter towards A ; but on 

 freezing again it must resume its original position. A or now gets 

 the heat, melts, oscillates, and freezes in its old position ; and so on. 

 From such heat there is plainly no molecular displacement. 



If Mr. Croll would make his theory a little more subtle, and re- 

 present as synchronous the freezing of B and the melting of A, he 

 would get some good of the sun's heat. For B, on melting, would 

 get as near A as possible, and as it swelled in freezing, A woidd 

 contract in melting, and hence B would remain in its new position. 

 Thus viewed, my difficulty number one disappears, for liquid B 

 would come down with an impact on A, and hence would more 

 readily melt A than any other molecule. This new reading of Mr. 

 Croll's theory has occurred to me since I started to write, and has 

 disposed me much more favourably to it. I can now see how the 

 sun's heat may keep down the glacier without getting in at the end 

 and travelling up it. This was my difficulty number two. It must 

 be borne in mind, however, that the distance the molecules can move 

 at a time in this way is the smallest imaginable. 



When I get to the paragraph on " Crevasses," I meet with more 

 difficulties. He says : " Suppose a change of inclination from, say, 

 4° to 8° in the bed of the glacier. The molecules on the sloj3e of 

 8° will descend more rapidly than those above on the slope of 4°. 

 A state of tension will therefore be induced at the jDoint where the 

 change of inclination occurs. The ice on the slope of 8° will tend 

 to pull after it the mass of the glacier moving more slowly on the 

 slope above. The pull being continued, the glacier will snap 

 asunder the moment that the cohesion of the ice is overcome." To 

 me this "explanation" is not quite "satisfactory." How does Mr. 

 Croll know that molecules will descend more rapidly on a slojDe of 



