GLACIAL MOTION. 49 



for, wherever the slope of the glacier's bed increases, cre- 

 vasses are formed by the increased strain to which the ice 

 is subjected. Crevasses are also formed in rapidly-moving 

 glaciers by the slight strain occasioned by the more rapid 

 motion of the middle portion. Still, in the words of Tyn- 

 dall, " it is undoubted that the glacier moves like a viscous 

 body. The centre flows past the sides, the top flows over 

 the bottom, and the motion through a curved valley cor- 

 responds to fluid motion."* 



To explain this combination of the seemingly contra- 

 dictory qualities of brittleness and viscosity in ice, physi- 

 cists have directed attention to the remarkable transfor- 

 mations which take place in water at the freezing-point. 

 Faraday discovered in 1850 that " when two pieces of 

 thawing ice are placed together they freeze together at the 

 point of contact, f 



" Place a number of fragments of ice in a basin of 

 water and cause them to touch each other ; they freeze 

 together where they touch. You can form a chain of 

 such fragments ; and then, by taking hold of one end of 

 the chain, you can draw the whole series after it. Chains 

 of icebergs are sometimes formed in this way in the arctic 

 seas." % 



This is really what takes place when a hard snow-ball 

 is made by pressure in the hand. So, by subjecting frag- 

 ments of ice to pressure it is first crumbled to powder, 

 and then, as the particles are pressed together in close 

 contact, it resumes the nature of ice again, though in a 

 different form, taking now the shape of the mould in 

 which it has been pressed. 



Thus it is supposed that, when the temperature of ice 

 is near the melting-point, the pressure of the superincum- 

 bent mass may produce at certain points insensible disin- 

 tegration, while, upon the removal of the pressure by 



* Forms of Water, p. 163. f Ibid., p. 164. % Ibid., pp. 164, 165. 

 5 



