EXPERIMENTS ON THE PLASTICITY OF ICE. 
165 
The theodolite, placed at Q, was pointed with its vertical wire on the well-defined 
angle of an erratic block Ql on the opposite eastern bank of the glacier, above Les 
Echellets (see Plate VIII. fig. 2.). By causing the telescope to traverse in a vertical 
circle, a transverse line joining the points Q, Ql was determined, and several stations 
were fixed in the compact ice eastwards of Q, at distances from it of 30, 60, and 90 
English feet and subsequently at 120 and 180 feet. These were numbered in succes- 
sion (1), (2), (3), (4), (5), and the permanence of their positions in the ice was secured 
as before by carefully driving vertical holes two feet deep, which were occasionally 
deepened, and covered with flat stones when not in use. As these points were in suc- 
cession nearer to the centre of the glacier, they were expected to move with gradually 
increasing velocity in advance of the imaginary line Q, Ql drawn across the ice. 
But as the theodolite stationed on the glacier at Q must partake of its motion 
whilst the mark Ql on the bank remained at rest, the visual line QQl would appear 
to revolve towards the origin of the glacier, and hence the relative advance of the 
points (l), (2), &c. would seem too rapid. To estimate the correction for this error 
the velocity of the glacier at Q must be determined, and also the distance QQl. For 
the former purpose the following method was adopted. When an observation at 
station Q had been completed, by pointing the telescope on Ql and observing the 
apparent advance of the points (1), (2), &c., the telescope was reversed in the Y’s, or 
turned 180° towards the western moraine, upon which it indicated from day to day 
a new position, owing to the angular revolution of the line joining the fixed point Ql 
and the moveable point Q. The point Q2 in the topographical sketch (Plate VIII. fig. 1 ) 
indicates the point where the visual line touched the moraine at the commencement 
of the observations on the 9th of August 1844. By the application of a scale or a 
similar method, the apparent advance of Q referred to the moraine Q2 was regularly 
measured. It is thus obvious that these apparent motions were too great (by the 
property of diverging lines) in the ratio of the distance Ql Q2, to Ql Q : and hence 
it became necessary to ascertain the position of Q2 as well as Ql. For this purpose 
a base-line of 300 feet was measured on the ice parallel to the length of the glacier, or 
perpendicular to the transverse visual line, extending from the point marked (3) to 
the point Q3 in fig. 3, whence by the theodolite the following bearings were taken : — 
Ql 0 0 
(3) 83 6 
Q2 148 0 
From which we deduce the distance from 
Ql to (3) . . . 2479 feet. 
(3) to Q2 . . . 640 feet. 
But as (3) is 90 feet east of Q, we have 
Q Ql =2569 feet. 
Q 
Q2 = 550 feet. 
