158 PHYSIOGRAPHY. [chap. 



movement of the glacier being perhaps only a few inches 

 or, at most, a yard or two, daily. 



This sluggish motion of a glacier, and the way in which it 

 accommodates itself to all the inequalities of the surface 

 over which it travels, long ago gave rise to the supposition 

 that ice is a plastic or viscous subftance, something like 

 dough or even treacle, so that it can sink into a depression, 

 or ride over a ridge, without losing its continuity. Yet, as 

 a matter of fact, ice is so brittle that if you pull, or try to 

 bend, it, it will snap, without stretching to any appreciable 

 extent. How, then, can the apparent plasticity be recon- 

 ciled with the undoubted brittleness ? Prof Tyndall ' has 

 shown the way out of this difficulty. 



When a schoolboy makes a snowball, he squeezes a hand- 

 ful or two of light snow into a hard compact lump ; and it is 

 worth noting that, if the snow be just on the point of thaw- 

 ing, he will be able to weld it into a firmer mass than if he 

 employed perfectly hard and dry snow. Snow, as we have 

 seen, is nothing but a confused mass of ice-crystals j and 

 the snowball becomes hard, partly, because it contains less 

 air, and, partly, because the little pieces of ice of which it 

 is composed, instead of remaining loose, stick firmly to one 

 another. But why do they thus become welded together? 

 Experiment shows that when two pieces of damp ice are 

 pressed together, they immediately freeze into one solid 

 mass. Faraday observed this curious fact five-and-twenty 

 years ago, and the phenomenon has been termed rege/afion. 

 Hence, when snow is strongly squeezed, the icy particles 

 freeze together into a compact substance ; and, hence, the 

 snow from which a glacier takes its birth, is pressed by the 

 weight above into a hard mass, more or less like true ice. 



1 See The Glaciers of the Alp, by John Tyndall, F.R..S. i860. 



