158 Mr. R. Mountford Deeley's Theory 



which will be of great value to us further on. Take a plate 

 of steel, say twenty-four inches long, three inches deep, and 

 half an inch thick. Firmly fix one end to a suitable support 

 so that the steel plate shall form a girder with its greatest 

 depth in a vertical position. Then distribute a number of 

 weights along the length of the bar. It at once becomes 

 deflected; that is shear, elastic shear, is produced in a manner 

 somewhat analogous to that in the pile of notepaper. We 

 will now T drill a row of holes along the plate; and when this 

 has been done, the girder, having been weakened, will be found 

 to have taken a still greater amount of set. Still further in- 

 crease the set by drilling several rows of holes. So far all 

 the operations have been possible ones; but I must now draw 

 upon the imagination somewhat, and perform operations which 

 cannot be carried out in practice. Take the material removed 

 from the numerous perforations in the plate, and replace it so 

 that the plate becomes whole again. It is evident that though 

 again solid, only that metal which formed part of the original 

 perforated plate is in a state of strain, that filling the holes 

 taking no share of the load. We will again drill a number 

 of holes, this time in the spaces between the older perfora- 

 tions, and another increase will take place in the deflection of 

 the plate. A strain will also be put upon the metal in the 

 first series of holes bored; and, in addition, a greatly increased 

 strain thrown upon what remains of the original plate. By 

 repeating the operation the girder could be deformed to any 

 desired extent, and, if necessary, such a violent strain thrown 

 upon any one point that local rupture would ensue. The 

 shear we have produced, and also that which took place in 

 the pile of paper, can now be compared with that taking place 

 in an ice-stream. Place the steel plate horizontally on a map 

 of a glacier drawn to a suitable scale, the end which was used 

 as a support being placed near the edge of the ice, and the other 

 end, which carried the weights, near the centre of the glacier. 

 The differential motion Tyndall so carefully measured will then 

 be found to be similar to, if not congruent with, the shear 

 which the steel plate has undergone. It must be remembered 

 that the ice is a girder of great depth, and the shear surfaces 

 are therefore approximately plane, whereas in the experimental 

 plate they were surfaces of considerable curvature. What is 

 the bearing of the knowledge thus gained upon the question 

 of glacier-motion ? 



Ice, though hard and brittle, is still an elastic substance. 

 When in mass it undergoes a certain amount of deformation 

 like any other body, by virtue of its own weight and elasticity. 





