of Glacier Motion. 157 



still worse; but I venture to think that, as the heat of former 

 discussion has now dissipated itself, and no theory has suc- 

 ceeded in standing the test of time, another attempt is justi- 

 fiable. I do not underrate the splendid work done by Rendu, 

 Forbes, Tyndall, Thomson, and many other eminent men; for, 

 as I have previously indicated, to them we owe a series of 

 observations on glacier-motion and structure which will 

 always rank high among the scientific achievements of the 

 present century. Still, the exact details of the process which 

 goes on within a glacier as it slowly changes its form, 

 accommodates itself to the ever-varying shape of its rocky 

 channel, and moves downwards with a differential motion 

 under the action of gravity, is still legitimate matter for 

 speculation. 



The theories now in the field are too well known to render 

 any detailed reference to them necessary ; I will therefore 

 content myself with stating, as simply as possible, what may 

 be regarded as a purely mechanical explanation of the ob- 

 served facts. 



It should be borne in mind that all the changes of form 

 which a body undergoes, provided no increase or decrease of 

 bulk takes place, may be regarded as due to what is called 

 simple shear. Take a pile of notepaper, or a pack of cards, 

 and displace it a little so that instead of standing vertically it 

 forms an inclined heap. Change of form has taken place 

 without change of bulk. The distortion has been accom- 

 plished by the shearing of the pile in planes parallel and co- 

 incident with the sheets of paper or card. Instructive ex- 

 amples may be seen in the quires of notepaper piled up in 

 various forms in shop windows. Every change of outline 

 suffered by a glacier, if we disregard melting and the small 

 internal changes of bulk produced by pressure &c, is due to 

 a shear of ice plane over ice plane. The resistance offered to 

 the sliding of the particles of one plane over the particles of 

 another plane is called friction; and the resistance they offer 

 to being torn asunder is called cohesion. Now the rigidity, 

 elasticity, &c, of a substance will depend upon the qualities 

 and values of these two properties. For instance, cast iron 

 breaks when put in tension without pulling out to any extent ; 

 for the cohesion of its particles is not sufficient to overcome 

 internal friction, and allow much more than elastic shear to 

 take place before breaking. On the other hand, the cohesion 

 between the particles of wrought iron is so great that it 

 stretches out — that is shears — and becomes heated by the re- 

 sulting friction. 



We are now in a position to understand an experiment 



