On the Causes of Glacier-Motion. 



•00002856 (Moseley, " Phil. Mag.," January, 1870), which is very high. 

 When a mass of ice, such as a glacier, suffers a rise in temperature, 

 either through conduction or radiation, it will expand ; this expansion 

 will take place mainly in the direction where movement is easiest, 

 that is, down the valley. If from any cause the temperature falls, 

 the glacier will again contract ; but since the expansion is assisted by 

 gravity whilst the contraction is opposed by it, the latter will be some- 

 what less in amount than the former, and when the ice has returned 

 to its original temperature its centre of gravity will have moved a 

 certain small distance down the valley. By such alternate expansions 

 and contractions the glacier moves gradually from the top to the 

 bottom of its course. 



That variations of temperature do take place in a glacier cannot 

 be doubted, whatever be the condition in which it lies. This granted, 

 the fact that it should move in the way described appears to me 

 no more surprising than that the sheets of lead on which Canon 

 Moseley made his well-known experiments did so move ; and that 

 the motion thus produced is of the character which answers to all 

 the facts of the case, so far as they are at present known, can, I 

 believe, be established. 



The controversy occasioned by Canon Moseley's articles was un- 

 fortunately terminated by his illness and death, before the matter 

 had been fully cleared up. The main objections urged to his theory 

 were two. The first was that a glacier is not one continuous 

 body (as assumed by Canon Moseley in his mathematical investiga- 

 tion), but is broken up into many parts by crevasses. But in the first 

 place, the assumption above mentioned is merely one of convenience, 

 and not in the least necessary to the theory. A detached piece of ice 

 would move in the same way as a glacier, or as the sheet of lead did 

 in Canon Moseley's experiments. Secondly, if a glacier is any- 

 where divided in its whole thickness by a crevasse, this is absolutely 

 fatal to the gravitation theories, since there can be no pressure 

 between the portions above and below this division. The only 

 possible explanation of crevasses, on these theories, is that they are 

 due to the glacier bending over a convex part of its bed. In that 

 case the bottom half will be in compression, and only the top half in 

 tension, so that the crevasse cannot possibly extend more than half 

 way through the thickness. 



The second objection was that the conductivity of ice is low ; hence 

 the effect of the heat would be confined to the layers near the surface, 

 and could not account for the motion of the glacier as a whole. This 

 objection does not seem to be confirmed by careful reflection upon the 

 way in which such forces act. Let us suppose a glacier 100 feet deep, 

 of which each successive foot expands and contracts alike throughout, 

 but adheres with a definite shearing resistance to the layers above and 



