80 VISCOUS THEORY OF GLACIER MOTION. [1845. 



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 (Travels 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 appears to have 

 the same origin : and in very weak currents moving through 

 very smooth and uniform channels (as the chiselled 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, 

 depending 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 par- 

 ticles is, however, scarcely affected by such a circumstance, for 

 the differential velocities which cause the ripple and the sepa- 

 ration are always small compared to the absolute velocity of 

 the stream ; and thus a floating body on the water (just as the 

 moraine on the glacier) perseveres in its course parallel to the 

 side with scarcely any perceptible disturbance. When, however, 

 the descent is violent and the friction great, floating bodies are 

 gradually drawn towards the centre, and this happens also in 

 exactly the same circumstances to the moraine of the glacier. 

 Plate II. figs. 3 and 4, show the relation of the ripple-marks 

 to the channel of a very flat smooth gutter in one of the side 

 streets of Pisa, sketched after heavy rain. 



These ripple-marks in water are well seen near the piers of 

 a bridge, or when a post is inserted in a stream and makes a 

 fan-shaped mark in the water cleft by it: such marks have 



