144 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
material already mentioned, which is retained there by a sluice or partition C which 
may be withdrawn at pleasure. 
The surface of the pool abed is then pretty thickly dusted over with a coloured 
powder, and the sluice is withdrawn. 
The pasty mass subsides slowly under its own weight into the lengthened form 
efgh. The film of colour on the surface is therefore broken up so as to cover three 
or four times the surface it did at first ; and its new distribution marks the lines of 
greatest separation of the superficial particles of the mass. The appearance of such 
a model when run is shown in fig. 2, and it manifests in the plainest manner the 
twofold tendency to separation in such a case where the channel is narrow and con- 
fined, and there is a certain mass of matter in front. Plate V. shows a more accu 
rate drawing taken from such a model. 
The lines of sliding separation occur most distinctly marked near the sides, where 
the friction is greatest, and the central parts are forced past the lateral parts, on ac- 
count of the less embarrassed and consequently swifter motion of the centre ; and 
they incline to the centre although the breadth of the channel be perfectly uniform. 
But the forces which tear asunder the parts (when such exists) act perpendicularly to 
the former and produce dislocations and fissures, which perfectly correspond to the 
direction and appearance of the crevasses of a glacier, that is, they are convex up- 
wards or towards the origin of the glacier. It is the former of these lines of separa- 
tion, or differential motion , which constitute and trace out with an exact parallelism 
the veined structure which I have described as forming the normal structure of all 
true glaciers. Plate V. is a representation of a very beautiful plaster model of more 
consistence than the other, in which the swelling of the surface and the direction of 
the open cracks produced by direct thrusts are most beautifully shown ; and are even 
more so in the model than in the engraving. The fissures are transverse and slightly 
convex to the origin in the higher part of the glacier, then gradually turning round 
they radiate from a centre in the lower part, exactly as in the glacier of Arolla (Tra- 
vels in the Alps, Plate VI.), and in all similar cases. 
The experiment above detailed was suggested to me by studying the ripple of 
streams of water, which appear to have the same origin : and in very weak currents 
moving through very smooth and uniform channels (as the chiseled sides of water 
conduits) the same may be made manifest by throwing a handful of light powder on 
the surface, which then becomes divided into threads of particles inclined in the 
manner I have described at a certain angle from the side towards the centre, depend- 
ing on the velocity of the stream. 
The slightest prominence of any kind in the wall of such a conduit, a bit of wood 
or tuft of grass is sufficient to produce a well-marked ripple-streak, from the side 
towards the centre, depending upon the sudden and violent retardation of the lateral 
streamlets and the freer central ones being momentarily edged away from them. 
The general course of the motion of the particles is, however, scarcely affected by 
