li. M. Deeley — Glacier Motion. 17 



has misunderstood my meaning in some cases and in others his 

 conclusions are unsound. 



The theory propounded in 1888 is as follows: "Take a plate of 

 steel, say 24 inches long, 3 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. . . . 

 We will now 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 increase 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 portion 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 perforations, 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." 



I also illustrated the theory by pointing out that the liquid cavities 

 produced in ice by the sun's rays would have a similar effect upon the 

 ice. I never, however, suggested or imagined that the few small 

 cavities near the surface of the glacier which could be formed by the 

 sun's heat were the cause of glacier motion, nor did I suggest, as far 

 as I can remember, that a glacier would not move if the temperature 

 were below the freezing-point. This seems to have been assumed 

 because I refer to the "efficacy of liquefaction and recongelation to 

 produce glacier motion ". 



Sir Henry Howorth also refers to a paper communicated by me to 

 the Geological Magazine for 1895. In this paper it is made clear that 

 liquefaction and recongelation are regarded as molecular phenomena. 

 In the case of a non-viscous solid the molecules or atoms may be 

 regarded as linked to each other in such a way that their motions, due 

 to the heat of the mass, never sever the linking. It may be that 

 when the body is strained sufficiently the vibrations of the molecules 

 do cause a severance of the molecular attachments ; but other attach- 

 ments are immediately formed and the body is permanently altered in 

 form without fracture. In other words the substance is a plastic one 

 and can only be permanently deformed when the stress exceeds 

 a certain figure. In the case of a viscous body the molecular linking 

 is being constantly broken here and there by the vibrations of the 

 molecules. Each link broken acts like the drilling of the hole in 

 the steel plate and leads to a change of form; for the freed molecules 



DECADE V. — VOL. VIII. — NO. I. 2 



