498 Rev. J. F. Blake— On the Motion of Glaciers, 



the solution of the problem, the peculiar circumstances under which 

 a glacier is placed, and the details of its motion. Knowing these 

 things only at second-hand, it would be folly in me to pretend to 

 give a solution, and I would only venture a few theoretical and 

 reconstructive remarks. 



Although the temperature of glacier-ice is not throughout as much as 

 82^ Fahr., a considerable portion is doubtless not far removed from 

 that, and we may perhaps conclude that motion only takes place in 

 that which is within 1^ of melting point. Perhaps some one acquainted 

 with glaciers could answer this ? Now it has been shown by care- 

 ful experiment that it requires more units of heat to raise ice through 

 the last degree of temperature before melting point than any other, 

 which can be accounted for by supposing some of the heat to be 

 taken up in preparing it to melt so to speak, in other words in 

 beginning the alteration of its molecular an-angements so as to make 

 the molecules more free to move upon each other. The settling 

 down of the whole mass of ice into a temperature state in which 

 this loosening of the molecules extended through some distance 

 would be more easily accomplished in a glacier than in a smaller 

 mass of ice. and in summer it would be more completely brought 

 about than in winter. It is true that Prof. Moseley has performed 

 experiments to show that ice, in order to separate, requires a shear- 

 ing force of 70 lbs. per square inch, and that the loosening would 

 have to be enormous to bring this within the power of gravity as 

 exercised on an ordinary glacier ; but every little helps. Another 

 way in which the shearing force of glacier-ice may be reduced is by 

 its containing a considerable quantity of air entangled within it. A 

 very interesting article on this point was contributed by John 

 Aitken to Nature, February 13, 1873, in which the writer details 

 experiments showing that such ice will bend and change its shape 

 under 6 lbs. or 7 lbs. pressure only per square inch ; so that the 

 nature and origin of the ice is obviously of considerable importance 

 in the question. 



I have myself tried similar experiments to those of Prof. Moseley, 

 so arranged that considerable pressure was brought to bear on the 

 surfaces to be sheared, and came to the conclusion that if you want 

 to see the ice shear, as much force must be brought to bear as he has 

 indicated ; but the force required proved to be so considerably depen- 

 dent on the time that I found it beyond my power to keep the ice at 

 the same temperature and diameter long enough to reduce the neces- 

 sary shearing force as low as I believed it might be reduced, if we 

 gave it time to do its work slowly and imperceptibly, as in a glacier; 

 and Mr. Miall has shown that substances whose shearing was greater 

 than their breaking strain may yet be bent, i.e. sheared, by a com- 

 paratively small pressure long applied. 



When we thus see how much the shearing force may be reduced by 

 time and temperature, and take also into consideration the effect of 

 alternate contraction and expansion with the variation of the external 

 temperature, we shall have data enough out of which to construct a 

 true theory of the motion of glaciers, without having recourse to 



