416 Mr, W, Hopkins [Jan. 31, 



to its molecular mobility as above explained. No glacialist, Mr. Hop- 

 kins conceived, could at present doubt the fact of the sliding motion. 



And yet, though nearly the same evidence had existed of the fact 

 in question, and the above explanation of it had been given upwards 

 of fifteen years ago, both the fact and the explanation had, till a recent 

 period, been almost entirely ignored. 



But the sliding of a glacier was not only important as an integral 

 part of its motion, but it also increased the internal pressures and 

 tensions, which were thus enabled to overcome the cohesive power 

 of the mass, to dislocate it, and leave it free to move onwards when its 

 motion might otherwise have been arrested. The action of the bed 

 of the valley on the bottom of the glacier had this apparently anoma- 

 lous property — that while it prevented the mass from moving with 

 more than a very slow uniform velocity, it exerted a force to hold it 

 absolutely at rest little greater than if the plane had been perfectly 

 smooth, and the glacier had moved with perfect freedom. It was so 

 far like the force of resistance aiforded by water to the descent of a 

 heavy body within it. Such resistance, while it had no power to hold 

 the body at rest, soon increased with the motion sufficiently to coun- 

 teract almost entirely the accelerating effect of gravity, and to reduce 

 the body very approximately to a uniform velocity which it could 

 never exceed. And such was the analogous case of the glacier. To 

 hold the heavy body in the water at rest would require a force equal 

 to the excess of the weight of the body over an equal volume of water ; 

 and so to hold any portion of a glacier at rest would require a force 

 nearly equal to the resolved part of its weight along the inclined plane 

 of its valley : whereas, if its motion were impeded by a large amount 

 of friction on its bed, it would require a far less force to hold it at 

 rest, and it would be capable of exerting only a far less force on any 

 obstacle impeding its onward progress. Thus, suppose the motion of 

 the glacier to be more or less impeded by the narrowing of its valley, 

 an enormous pressure would be produced on the impeded portion of 

 the mass, by the tendency of the less impeded portion behind to press 

 forward, and the more freely this latter portion should slide over its 

 bed, the greater would manifestly be the pressure which would be 

 thrown on the impeded portion before it. We might thus see how 

 enormously the sliding of the glacier might increase the fracturing 

 power of the internal pressures and tensions, and help to preserve the 

 continuity of the process of alternate dislocation and regelation on 

 which the continuity of the mass and structure of the glacier and that 

 of its motion depend. 



According to the views which Mr. Hopkins had been endeavouring 

 to develope, if we conceived a glacier to be placed in its valley at 

 rest and in a state of no constraint, it would begin to move, partly 

 by sliding on its bed and partly by virtue of that small relative dis- 

 placement of the molecules of the mass of which all bodies are sus- 

 ceptible in some degree, without dislocation. The whole glacier 

 would thus become a continuous mass in a state of constraint, and 



