PLASTIC MODELS. 
145 
such a circumstance, for the differential velocities which cause the ripple and the 
separation, 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 IV. figs. 3 and 4, shows 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 been much neglected by writers on hydraulics; but in one of the 
most ancient hydraulic treatises, that of Leonardo da Vinci, lately printed from the 
MS. in the Italian collection of writers on hydraulics, they are very well described 
and figured. A case parallel to the last-mentioned, where a fixed obstacle cleaves a 
descending stream and leaves its trace in the fan- shaped tail, is well seen in several 
glaciers, as in that at Ferpecle, and the Glacier de Lys on the south side of Monte 
Rosa, particularly the last, where the veined structure follows the law just mentioned. 
And I desire here to record that the views just presented as to the origin of the 
veined structure of ice, were confirmed, but were not suggested, by the experiments 
on viscous fluids just mentioned. The necessity of the tearing up of a solid mass, if it 
moved at all in a bed presenting insurmountable resistances on all sides, in direc- 
tions such as the veined structure presents, was foreseen by me whilst dwelling 
amongst the glaciers themselves, at a distance from books or the means of experi- 
ment. The followingextract from my Third Letter to Professor Jameson, written in 
1842 from the remote village of Zermatt, contains the substance of all that I have 
since developed and illustrated at greater length and in different ways rather to meet 
the difficulties of others, than to confirm what was plainly fixed in my own mind. 
“The glacier struggles between a condition of fluidity and rigidity. It cannot 
obey the law of semi-fluid progression (maximum velocity at the centre, which is no 
hypothesis in the case of glaciers, but a fact), without a solution of continuity per- 
pendicular to its sides. If two persons hold a sheet of paper so as to be tense, by 
the four corners, and one move two adjacent corners, whilst the other two remain at 
rest or move less fast, the tendency will be to tear the paper into shreds parallel 
to the motion ; in the glacier the fissures thus formed are filled with percolated 
water, which is then frozen. It accords with this view, — 1st, that the glacier moves 
fastest in the centre, and that the loop of the curve described coincides (by observa- 
tion) with the line of swiftest motion. 2nd. That the bands are least distinct near the 
centre, for there the difference of velocity of two adjacent stripes parallel to the 
length of the glacier is nearly nothing ; but near the sides, where the retardation is 
greatest, it is a maximum. 3rd. It accords with direct observation that the differ- 
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