172 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
daring the continuance of these experiments, nothing can be so irksome as the ne- 
cessity of persevering in the face of physical obstacles, the only alternative which the 
necessary limitation of my stay afforded being to abandon them. I do not speak of the 
painful effort of conducting delicate observations for hours under a hot sun, whilst the 
feet are immersed in the liquid sludge of decaying snow, for I am not aware of having 
sacrificed the precision of a single observation to such a cause (though in the course 
of my glacier experience I have sometimes been compelled to abandon or discontinue 
observations), but it is easy to see that the success of experiments like this depended 
upon the absolute fixity of the marks inserted in the unstable and wasting surface of 
the glacier, and that the most dry and uniform condition of the ice seemed alone to 
promise a chance of finding the small pins in the exact positions in which they had been 
planted a day or two before. Instead of this, eleven out of nineteen days which I 
spent at Chamouni were wet, and notwithstanding the season of the year, the glacier 
was repeatedly covered with snow, which in melting under a succeeding fierce sun, 
left the surface honeycombed by infiltration and streaming with wet, so that the pre- 
servation of the holes was only effected by laboriously covering every one with large 
flat stones during the intervals of observation, and even this was not free from other 
disadvantages which it would take too long to particularize. On the whole, the uni- 
formity of the triple curves exhibited in fig. 2 is surprising, considering the local 
errors to which the fixation of the pins was liable, and the smallness of the quanti- 
ties sought. The first two curves, those for the 21st and 23rd August, are indeed as 
perfectly regular as it would be possible to expect from this kind of observation, even 
much more so than I had ever hoped to attain ; but on the 26th the holes containing 
the pins were more degraded, and some manifest errors have arisen from this cause, 
and evidently affect only single marks, such as the twelfth and twenty-fifth mark, 
which singly have inclined forward or backward by the fusion of the ice. With 
these preliminaries as to the reasons why the irregularities of these curves should be 
judged with indulgence, I will state briefly the most apparent general results. 
First. The flexure of the ice is proportional, almost exactly to the time elapsed in the 
intervals of the observations ; and it is also graduated from point to point, not stac- 
cato, as would inevitably have been the case had the relative motion been due to 
the sliding of finite portions past one another, as in Plate VII. fig. 3. We perceive 
nothing of the kind. 
Secondly. A singular peculiarity strikes the eye, which at first puzzled me, but 
when the cause was explained, confirms in no slight degree the accuracy of the 
methods employed and their fitness to reveal the minutest motion of the glacier. The 
curves of Plate IX. fig. 2, cut the axis, not all exactly at the same point; but on 
an average this point may be fixed with tolerable accuracy between the third and fourth 
mark, or seven feet from the fundamental station Q. The Jirst and second marks 
moved evidently slower than the point Q, or the zero, and the progress of the third 
and fourth is dubious or irregular. The cause of this peculiarity was clearly ascer- 
