168 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
counted for by any casual justling or sliding of one finite portion of the ice past 
another, which would inevitably have left some of the points relatively at rest during 
some one of the many intervals of observation, and given to others evidence of a start- 
ing motion until friction had established a fresh position of repose amongst the 
struggling masses. 
Secondly. This Table enables us to establish not only the continuity of motion of 
any one point, but the continuity of the relation which connects the points (1), (2), 
(3), &c. For instance, the relative motions of (1) being 
•59 -56 *56, 
and those of (2) being 
117 M2 1-16, 
the ratios are 
1'98 2-00 2-07. 
In like manner the ratios between (3) and (2) will be found to be 
1-37 1-42 1-44. 
Thirdly. The flexure of the ice may be conveniently represented by a diagram, in 
which the several ordinates are set off corresponding to the relative spaces moved 
over. But to find the initial positions of the fourth and fifth marks, the proportional 
motion for the first period, when they were not observed, must be deduced from the 
comparative velocity of the period when the observations were comparable. Thus 
by Table II. the relative velocity of (3) to (4) during the time that they were ob- 
served together is 19"05 : 24’3 ; consequently whilst (3) moved over 26*9 inches (4) 
would have moved over 34*3 inches; the proportional motion for 16'75 days. In 
like manner for the mark (5) we have the simultaneous motions of (3) and (5) ex- 
pressed by 14‘6 inches and 26*5 inches, and hence by proportion, as before, we find 
14-6 : 26-5=26-9 : 48*8 inches, 
the relative motion of (5) in 16’75 days. 
From these data the simultaneous relative motions of these six stations may be pro- 
jected in a curve, or rather polygon, as shown in Plate IX. fig. 1. This is interesting, 
as showing very plainly, not only the regulated increase of swiftness of the glacier 
towards the centre, but that the variation of the variation is clearly brought out, in- 
dicated by a convexity in the direction of the motion, and confirming the general 
principle long ago announced by me, that the retardation is relatively greatest towards 
the side and less towards the centre. I appeal to any one conversant with the laws 
of mechanics in their practical application, whether the manifest continuity of such a 
law does not plainly include a continuity in the mutual action of the parts of the 
mass under experiment, and even independent of the manifest absence of great dis- 
locations, would not establish the doctrine of a molecular yielding, or plasticity in the 
ice as opposed to the irregular justling of great blocks, admitting that such could 
exist unperceived. 
