CAUSE OF GLACIER MOTION. 249 



the rate at which the glacier descends depends upon 

 the amount of heat which it is receiving. This shows 

 that the motion of the glacier is in some way or other 

 dependent upon heat. But in what respect can heat 

 be regarded as a cause of motion ? Heat cannot be 

 directly a cause of motion. Neither can heat produce 

 motion or displacement of the particles by making the 

 ice soft and plastic ; for we know that the ice of a 

 glacier is not soft and plastic, but hard and brittle. 

 Its proper function will be seen when considering the 

 bearing of Revelation on glacier motion. 



Whether or not regulation can be regarded as a 

 cause of glacier motion will depend on the view which 

 we may adopt as to the physical cause of regulation 

 itself. There are three theories which have been 

 advanced to explain regelation. 



According to Professor James Thomson's theory, 

 pressure is the cause of regelation. Pressure applied 

 to ice tends to lower the melting-point, and thus to 

 produce liquefaction, but the water which results is 

 colder than the ice, and refreezes the moment it is 

 relieved from pressure. When two pieces of ice are 

 pressed together, a melting takes place at the points 

 in contact, resulting from the lowering of the melting- 

 point ; the water formed, re-freezing, joins the two 

 pieces together. 



The objection which has been urged against this 

 theory is that regelation will take place under circum- 

 stances where it is difficult to conceive how pressure 

 can be regarded as the cause. Two pieces of ice, for 

 example, suspended by silken threads in an atmo- 

 sphere above the melting-point, if but simply allowed 

 to touch each other, will freeze together. Professor J. 

 Thomson, however, attributes the freezing to the pres- 

 sure resulting from the capillary attraction of the two 



