EXPERIMENTS ON THE PLASTICITY OF ICE. 
169 
The period through which this experiment extended (seventeen days) is conclusive 
against the idea that a small flexure could take place until the accumulated strain 
on the solid produced a rupture, which relieved the strain, and so forth, per saltum. 
The continuity of glacier motion in every case except that of precipitous descents or 
ice-falls, first proved by my experiments in 1842, is now universally admitted by those 
who have had any personal experience in the measurement of glacier motion, however 
opposed to my theoretical views*. The changes for seventeen days were connected 
(as has been shown) by a law of continuity established by numerous intervening ob- 
servations ; and the flexure or distortion of the ice amounted in this time to no less 
than four feet at the opposite ends of a line 180 feet in length. It is quite certain, 
from my own previous observations and those since made by M. Agassiz’s directions 
on the glacier of the Aar, that the movement thus shown to have continued seventeen 
days without a saltus would have continued the whole season in the same manner. 
In fact, the deformation or flexure thus observed being sufficient to account for the 
whole excess of the central above the lateral motion, is in itself an explanation, and a 
proof that the explanation is adequate, and leaves nothing residual to be accounted 
for by saltus 
I have more to add on this subject, but shall first give an account of an extension 
of this experiment on the actual flexure of the ice, upon so elaborate a scale as I 
scarcely ventured to hope would prove successful, especially as the time I could devote 
to watch its progress was small, and the circumstances of weather excessively un- 
favourable. 
Having succeeded so well with the thirty feet station in the transverse line, I 
thought of multiplying the points of observation still further, so as to obtain a polygon 
of flexure more nearly approaching to a curve. This I did by making the first ninety 
feet of the transverse line, i. e. the space between Q and (3), Plate VIII. fig. 3, the 
subject of more immediate experiment, fixing in it forty-five stations only two feet 
apart. After several partial failures, which gave me, notwithstanding, encouraging- 
results, I selected this plan. A space a foot wide and ninety feet long was cleared 
with hatchets and ice tools, so as to arrive at a nearly even surface of the hard deli- 
cately veined ice ; and gutters were made so as to drain as far as possible the surface 
water from the part under experiment. The theodolite being stationed as usual over 
Q, and the vertical wire of the telescope describing a great circle passing through the 
line QQl transverse to the glacier, an assistant (Balm at), directed by my signals, bored 
* See proofs cited in my Ninth Letter on Glaciers, Edinburgh Philosophical Journal, April 1845, and in 
second edition of Travels in the Alps, Appendix. 
t Mr. Williamson, Fellow of Clare Hall, Cambridge, to whom I proposed this experimental test of the 
theory of movement by echelons, made a series of independent observations on the Mer de Glace, which coin- 
cided in result with what has been stated above. After a patient examination of these facts, and of others 
which he observed on different glaciers, I am glad to say that Mr. Williamson was led to abandon the theory 
of sliding columns or fragments, and to accept that of plasticity as connected with the mechanism of the veined 
structure which I have endeavoured to illustrate above. 
z 2 
