Professor Forbes on the Leading Fhenomena of Glaciers. 243 



I have succeded in illustrating this theory by constructing 

 models of a viscid material (a mixture of plaster and glue, 

 •which does not set readily), poured down irregular channels* 



actually performing the experiment suggested in the text, of pouring plaster of 

 Paris and glue into a narrow rectangular box, and sluicing it up by a bit of wood, 

 removable at pleasure. The surface of the viscid mass was then strewed, whilst 

 level, with a coloured powder, and the sluice withdrawn. The liquid flowed ex- 

 actly as I have described, and the colouring matter was drawn out into threads, 

 precisely resembling, on a minute scale, in delicacy and continuity, the veined 

 appearance of the glacier surface. The explanation appears to be this ; — that 

 the velocity of the central strata tends to pull the lateral strata towards the centre, 

 as well as parallel to the length of the glacier ; this produces a slight lateral as 

 well as longitudinal discontinuity, for the actual motion of the side strata towards 

 the centre is exceedingly small, and (as the phenomena of moraines tell us, which 

 act like the floating cork in the experiment above described) does not sensibly dis- 

 turb the parallelism of motion of the parts of the ice. In short, the internal 

 movements are of an order so inferior to the general movement of the stream, that 

 they may probably be left out of account in describing the general movement, al- 

 though by the fissured structure which they induce, they have sufficient evidence 

 of their existence. But if the slope and consequent hydrostatic pressure be great, 

 the movement towards the centre may, as supposed in the text, be of an order to 

 modify appreciably the direction of movement of a particle. In an ordinary 

 liquid like water, the direction of t^e ripple marks, occasioned by the friction 

 of a stream in proceeding from a wider to a narrower channel, is an example of 

 the same thing, namely, lines of maximum mutual friction of the particles against 

 one another. They converge rapidly towards the centre of the stream, whilst the 

 motion of the fluid, indicated by a floating body, deviates but little from the direc- 

 tion of the axis of the channel. 



The same view, mutatis mutandis, explains the frontal dip and conchoidal form 

 of the bands between the top and bottom of the glacier. There is here also a drag 

 acting from the upper to the lower strata, and fissures are produced in consequence 

 of the sluggish lower strata refusing to follow the swifter upper ones. This may 

 also be a quantity of an order so inferior to the actual rate of motion of the ice, 

 as to make it inappreciable by direct experiment. 



The experiment, on a model described in this note, is more strictly analogotu 

 to the glacier phenomena, than those of a more striking kind described in the next 

 page, where the succession of colours naturally gives to the mind the impres- 

 sion of a primitive structure near the origin of the glacier, which is riechanically 

 defoi'med into these conchoidal surfaces. They strikingly recall, however, this 

 important fact, that the direction of maximum distension of the particles must be, 

 not parallel to the length of the glacier, but in the direction of the branches of 

 these elongated loops, since their elongation is the simple result of the mechani- 

 cal tension to which they are subjected ; hence, a motion in imraUel directions, 

 with unequal velocities, of a series of unorganized particles, confusedly ai-ranged, 

 must induce a mutual linear distension in a direction inclined to their real mo- 

 tions ; and this being unequal for adjacent portion!*; induces the delicately fibrous 



