174 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
leaving a vacuity behind. No such slip is recorded in the observations, and no such 
gap or vacuity was left during the continuance of the observations for seventeen days. 
Nothing approaching to an open fissure occurred in any part of the space of ice under 
observation, and the few flaws noted (see the column of remarks in Table IV., 
and the indications of them by dotted lines and arrows in Plate IX. fig. 2, showing 
their directions) were perfectly close, and all more or less of zigzag forms, pre- 
venting the possibility of sliding. The veined structure was developed in every part 
of the section, in some parts more admirably than others, as near the seventh mark, 
and between the fortieth and forty-fifth. These countless blue veins may be con- 
sidered as so many flaws or partial solutions of continuity, existing or having existed ; 
they are almost perfectly straight , and (as will be shown immediately) exactly in 
the direction in which the relative motion of the parts of the ice demonstrated by 
these experiments takes place. It would require strong evidence to convince us 
t hat these veins are not occasioned by, and the mechanism of, the plastic motion of 
the ice. 
The beautiful convexity of the curves in the direction of motion which the eye at 
once seized, and could more accurately distinguish by the theodolite, as having its 
largest sagitta at the twenty-fifth mark, namely, almost exactly midway between the 
fourth mark or convex origin, and the forty-fifth or extreme mark, contains in itself 
an evidence with which no person of correct habits of thought can fail to be struck 
as proving a regular plastic action of gravity or other propelling force acting from 
point to point on the mass of the glacier. I made however a check experiment 
almost unnecessary, but which I will here detail. Lest it should be alleged that the 
whole area under experiment was a moving one capable of being swung round by the 
pressure upon the centre of the glacier, so that the displacement of the transverse 
line was due to rotation of the mass operated on round some distant centre, I took 
care near the commencement of my experiment to fix a mark in the ice in the same 
line with Q parallel to the length of the glacier, or perpendicular to the transverse 
visual line at the commencement of the experiment. This point is marked q in 
Plate VIII. fig. 3. Now had the block of ice on which the marks Q, q, (1), (2), (3), &c. 
were fixed, been revolving round some fixed or moveable centre, the right angle (3) Q q 
would have remained a right angle, and so for (2) Q q and (1) Q q. But these angles 
constantly became more obtuse, and that in different degrees depending on the con- 
vexity of the curve Q, (1), (2), (3) : and so of (4) and (5). It is impossible, therefore, 
to avoid the conclusion, that the solid ice was itself distorted to the amount of the 
excess of progression of the more central above the lateral stations. 
The results are contained in the following Table, the five stations being those 
formerly mentioned at 30, 60, 90, 120, and 180 feet from Q. The first three stations 
were fixed when the visual line had an azimuth of 89° 55' from q; the fourth was 
fixed on the 14th of August, when the visual line had revolved through 3' (its daily 
progression towards q in consequence of the translation of the station Q being P 30" 
exactly), and the fifth on the 17th, when the visual line was in azimuth 89° 4/^'. 
