Mr. J. Croll on the Cause of the Motion of Glaciers. 169 



under another form : heat does not reduce the shearing-force of 

 the ice of a glacier to something like 1*3 193 lb. per square inch 

 of surface, the unit required by Mr. Moseley to enable a glacier 

 to shear by its weight ; the shearing-force of the ice, notwith- 

 standing all the heat received, still remains at about 75 lbs. ; 

 how, then, can the glacier shear without any other force than its 

 own weight pushing it forward ? This is the fundamental ques- 

 tion ; and the true answer to it must reveal the mystery of gla- 

 cier-motion. We are compelled in the present state of the pro- 

 blem to admit that glaciers do descend with a differential motion 

 without any other force than their own w r eight pushing them 

 forward; and yet the shearing-force of the ice is actually found 

 to be thirty or forty times the maximum that would permit of the 

 glacier shearing by its weight only. The explanation of this 

 apparent paradox will remove all our difficulties in reference to 

 the cause of the descent of glaciers. 



There seems to be but one explanation (and it is a very obvious 

 one), viz. that the motion of the glacier is molecular. The ice 

 descends molecule by molecule. The ice of a glacier is in the 

 hard crystalline state, but it does not descend in this state. 

 Gravitation is a constantly acting force ; if a particle of the ice 

 lose its shearing-force, though but for the moment, it will de- 

 scend by its weight alone. But a particle of the ice will lose its 

 shearing-force for a moment if the particle loses its crystalline 

 state for the moment. The passage of heat through ice, whether 

 by conduction or by radiation, in all probability is a molecular pro- 

 cess ; that is, the form of energy termed heat is transmitted from 

 molecule to molecule of the ice. A particle takes the energy from 

 its neighbour A on the one side and hands it over to its neighbour 

 B on the opposite side. But the particle must be in a different 

 state at the moment it is in possession of the energy from what it 

 was before it received it from A, and from what it will be after it 

 has handed it over to B. Before it became possessed of the energy, 

 it was in the crystalline state — it was ice ; and after it loses pos- 

 session of the energy it will be ice ; but at the moment that it 

 is in possession of the passing energy is it in the crystalline or 

 icy state ? If we assume that it is not, but that in becoming- 

 possessed of the energy, it loses its crystalline form and for the 

 moment becomes water, all our difficulties regarding the cause of 

 the motion of glaciers are removed*. 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 ? 



* See Phil. Mag, for March 1869, p. 201. 

 Phil Mag. S. 4. Vol. 40. No. 266. Sept. 1870. N 



