162 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
Our object is, in this section, merely to state the view in its most plausible form, 
which in the succeeding - section we shall controvert by experiments giving - it a di- 
rect negative. In the third portion of this essay we shall enter more at large into 
the phenomena of crevasses, and mention other objections to this hypothesis and every 
modification of it. 
According to this view, the friction of the ice against the sides of the valleys will pro- 
duce a dislocation of the glacier into longitudinal stripes (as shown in Plate VII. fig. 1.*), 
where a transverse line bb' becomes by the irregular motions of the ice distorted into 
the zigzag form hcc'h. Or if we suppose the plasticity of the ice to be sensible, but 
that its action is accompanied with fractures, the abruptness of the angles of the 
figure will be softened, as in the broken line Irnm'l in the lower part of the same 
figure. This latter hypothesis evidently merges into the true plastic theory, when 
the part of the progression due to the flexure of the transverse lines bears a large 
proportion to the effect of the longitudinal slide, or more generally, when the surfaces 
of sliding or yielding become greatly multiplied, when the notched line will merge 
into a curve. 
The passage of the glacier through a gorge or contraction is explained on the same 
view by figure 2, where the resistance of the sides having occasioned a series of 
parallel longitudinal rents as before, the portion of the glacier beyond the limits of 
breadth of the gorge BB' is supposed to be detained or embayed whilst the interme- 
diate columns slip through. 
§ 5. Experiments at Chamouni on the Plasticity of Ice. 
It has been shown that in order to reconcile De Saussure’s theory of sliding mo- 
tion with the ascertained fact that the centre of the glacier moves faster than the 
sides, it had been assumed that solutions of continuity or longitudinal crevasses 
were formed parallel to the length of the glacier, by means of which the central por- 
tion slides past that adjacent to it, and so on for successive strips as we approach the 
sides, the more rapid retardation near the sides being rendered mechanically possible 
by the increased number of these longitudinal disloeations. 
The result was therefore predicted to be that the glacier would be found to move 
by echelons, or that strips of ice of a certain number of feet, or yards, or fathoms, 
would move either suddenly or by gradual sliding, but at all events so as to mark by 
an abrupt separation at the longitudinal fissure, that the one portion of ice has slipped 
past the other by a distinct measurable quantity. 
When I first learnt at Geneva, in August 1844, from Mr. Hopkins’s published 
papers-}-, that this was really the author’s meaning, it occurred to me that the proof 
* These figures and their interpretation are taken from Mr. Hopkins’s First Memoir in the Cambridge 
Transactions, vol. viii. part 1. A figure similar to the first is to be found in a more recent paper by the same 
author in the Philosophical Magazine for June 1845. 
f Cambridge Transactions, vol. viii. 
