EXPERIMENTS ON THE PLASTICITY OF ICE. 
173 
tained on the spot to be the existence of two crevasses belonging to the lateral system 
of crevasses, between which at their thinning out the station Q had been placed, under 
the idea that their distance was sufficiently great not to affect the motion. The 
position of these crevasses is shown in Plate VIII. fig. 3, by which it will be seen that 
they were fifty feet apart in the direction of the length of the glacier, and that a line 
joining their extremities passed eight feet nearer the centre of the glacier than the 
point Q, thus almost coinciding with the point of contrary flexure, which was evi- 
dently occasioned by a slightly superior advance of the mass of ice on which Q was 
placed, thus insulated in some degree between the two fissures. This enables us to 
transfer the origin of our curve to a point of undoubted solidity, namely, the fourth 
station, from which point it swells with the regularity which has been described, and 
which establishes the compactness of the ice experimented on in the most convincing 
manner. 
Thirdly . The curves reckoned from the origin, thus experimentally indicated, show 
in a beautiful manner the convexity in the direction of the glacier motion before 
alluded to, which is singularly striking, considering the shortness of the space in 
which it is developed with nearly mathematical precision, being only about -^th of 
the breadth of the glacier in this place (see ground plan, Plate VIII. fig. I .). Even an 
inspection of the curves (Plate IX. fig. 2) can faintly convey the impression made upon 
my own mind, when upon the 26th of August I placed the theodolite for the last time 
over station Q, and caused the vertical wire to pass in front of the line of pins bent 
into the convex shape by the relative motion of six days’ continuance. Thus seen in 
foreshortened perspective, the eye would in an instant have seized an abrupt motion 
or discontinuity of the line, but “the appearance of the curve they formed was beau- 
tiful ; the whole line of pins was deviated from the usual line QQl by an angle equal 
to 1245 inches, seen at a distance of ninety feet, or about 40', and besides this, the 
pins lay in a beautiful and nearly continuous curve, presenting its convexity towards 
the valley, and decidedly without any great step or start. This was beautifully seen 
when I directed the vertical wire of the theodolite upon the forty-fifth pin and caused 
it to describe a vertical plane*. I observed however a curious fact, plainly indicated 
by the numerical results ; the curve crossed the axis at the fourth pin, and attains its 
greatest convexity at the twenty-fifth -f-.” 
Fourthly. That no information might be wanting as to the precise condition of the 
mass of ice under experiment, I made a very minute examination of the state of the 
transverse line with respect to the occurrence of flaws in the ice. The most import- 
ant of these was one which returned into itself, crossing the line towards the origin 
of the glacier between the twenty-sixth and twenty-seventh marks, and returning 
backwards between the fortieth and forty-first without extending further upwards. 
Such a flaw, even if devoid of cohesion, could only act by allowing the piece of ice 
contained between the twenty-seventh and fortieth mark to slip bodily forwards, 
* An approximation to this effect will be obtained by stretching a fine thread over the figure. 
t From my notes made at the time. 
