The Theory of Glacier motion. 85 



of the glacier caused by the growth of the grains must be 

 very slight, if any. 



For these reasons and for the further reason that it 

 seems impossible to correlate the differential motion of 

 glaciers, as observed by Forbes and others, with any process of 

 mere general swelling of its bulk, it seems to me that we 

 cannot assign to this cause any but a slight influence in the 

 movement of glaciers. 



Let us now turn to another theory which involves dila- 

 tation and contraction in another form, namely, that of Mr. 

 Moseley. He had noticed that the lead upon the roof of a 

 church at Bristol gradually descended owing to alternate 

 variations of temperature, and arguing from this, he urged 

 that a glacier's motion was best explained by the alternate 

 expansion and contraction of the ice which forms it, due to 

 variations of temperature, which motion should take place 

 in the direction where it is easiest, namely, down the valley ; 

 where expansion would be assisted by gravity, while contrac- 

 tion would be resisted by the same force: thus expansion 

 would gain somewhat upon contraction in every alternation of 

 temperature, and the general centre of gravity of the mass 

 would move down somewhat. Mr. Moseley defended this 

 theory in several papers. 



In criticizing it, we must in the first place remember 

 that ice cannot expand with heat when above the 

 freezing point, and, if so, as Mr. Croll pertinently says, 

 how are we to account at all for the motion of glaciers 

 in summer on this theory. When the temperature of ice 

 is below freezing point, the rays which are absorbed will 

 no doubt produce dilatation ; but during summer, when the 

 ice is not below freezing point, no dilatations can take 

 place. All physicists agree that the rays that are then 

 absorbed go to melt the ice and not to expand it {Phil, 

 Mag., XL. 166). 



If Moseley's theory be correct we cannot understand 

 F 



