The Theory of Glacier motion. 93 



inodification of the theory of Sutclifife and of Thomson, 

 which I had better state in his own words. "It is found," he 

 says, "that the rate at which a glacier descends depends upon 



the amount of heat which it is receiving heat assists 



gravitation to shear the ice not by direct pressure but by 

 •diminishing the cohesive form of the particles, so as to 

 enable gravitation to push the one past the other. . . . There 

 seems to be but one explanation, namely, that the motion 

 of the ice is molecular. The ice descends molecule by 



molecule The passage of heat through ice, whether by 



conduction or radiation, is in all probability a molecular 

 process, that is, the form of energy termed heat is trans- 

 mitted from molecule to molecule of the ice, a particle takes 

 the energy from its neighbour and passes it on, but a particle 

 •must be in a different condition when in possession of the 

 energy, to what it is before and after. Before it was in a 

 <:rystalline state it was ice, and after it will be ice, but at 

 the moment it is in possession of the passing energy it 

 becomes water. We know that the ice of a glacier in the 

 mass cannot become possessed of energy in the form of 

 heat without becoming fluid. May not the same thing hold 

 true of the ice particle " {Phil. Mag., XL. 168 — 9). 



He urges, in effect, that the shearing force of the particles 

 of ice when heat is passing through them is not constant, and 

 ^* that while a molecule of ice is in the act of transmitting 

 the energy received, it loses for the moment its shearing 

 force if the temperature of the ice be not under 32° F." 

 •Consequently a molecule, directly it assumes the fluid state, 

 is completely freed from shearing force, and can descend by 

 virtue of its own weight without any impediment. All that 

 the molecule requires is simply room or space to advance 

 in. If the molecule were in absolute contact with the 

 adjoining molecule below, it would not descend unless it 

 could push that molecule before it, which it probably would 

 not be able to do. But the molecule actually has room in 



