144 Review of the Controversy Regarding the Motion of Glaciers. 



in succession, and reducing it to a lower level, so as to make room for 

 the descent of the next above, as soon as it has become liquid by the 

 receipt of energy from its resolidified predecessor, now again fixed by 

 cohesion at a lower level. But, we may ask, does the heat (supposing 

 it to enter the ice) enter at the lower end of a glacier and pass i;j9- 

 tvard toward its neve ? On the contrary, if it pass into the ice at all, it 

 must do so from the surface, and be transmitted vertically downward. 

 Consequently, the melted molecule can not descend before it passes on 

 its heat of liquidity to the next helow it. It has no room to fall in. Its 

 own contraction or melting will leave a space above it temporarily va- 

 cant. But the particle above it is already solid, and can not fall into 

 that cavity, and before it can pass on its energy to the molecule below 

 it, it must surrender that energy itself. The heat, and, consequently, 

 the liquidity, can not exist in two molecules at the same instant of 

 time. It must pass from the upper one before it can be possessed by 

 the lower one. But in passing from the upper one it must cause that 

 molecule to solidify exactly in the position it previously occupied. By re- 

 solidifying it would fill again the vacuity temporarily produced by its 

 liquefaction, and the same process would take place in the succeeding 

 particle. No downward motion, therefore, can be produced. The 

 molecules can not flow while in their supposed liquid condition, for 

 want of space, and when space is given to them by the liquefiiction of 

 the molecule below, they can not descend, because they are again 

 chained by cohesion, needing shearing force to overcome it, which is 

 not present. On a fair application, therefore, of Mr. Croll's own illus- 

 tration, his theory fails to account for a phenomenon which he ex- 

 plains by special pleading in one particular and favorable instance. 



Having thus discussed some of the most important phj^sical facts, 

 which jMr. Croll seems to have entirely overlooked, in their hearings 

 upon his theory, we propose now to follow it into some of its obvious 

 results, in order to determine how far it fulfills the first great con- 

 dition of every hypothesis — adequacy to account for the phenomena 

 observed. 



There can be no doubt that if we admit Mr. Croll's premise, that 

 successive layers of the ice become momentarily liquefied, and during 

 that short interval flow down to a lower level, we shall obtain a mo- 

 lecular movement of the glacier downward. This result is entirely due 

 to the W'cight of the molecule itself, unaided from without. Mr. Croll 

 does not admit or require any doAvnward pressure of the mass of the 

 glacier above it. Every particle flows in the glacier exactly as it 

 would flow in a river, with the single exception that its flow is inter- 

 mittent. It would follow, therefore, that the different parts of the gla- 



