ME. HOPKINS ON THE THEOEY OP THE MOTION OP OLACIEES. 
685 
tions for this purpose were made by Principal Foebes in 1846, at the bottom of the 
Glacier des Bois at Chamouni. He found that the velocity of a point on the surface, 
at the height of 143 feet above the bed of the glacier, was to that of a point 8 feet above 
the bed, in the ratio of nearly 16 : 10*, whence it follows that, if we divide the whole 
velocity of the surface into eight parts, five of them will be due to the motion of the 
bottom of the glacier, and three to that change of form of the mass by virtue of which 
its higher move faster than its lower portions. Dr. Tyndall has also made similar 
observations on the fiank of the Mer de Glace f, from which it appears that the motion 
of the upper was there rather more than twice that of the lower surface. These obser- 
vations may or may not determine approximately the average ratio between the velo- 
cities of the upper and lower surfaces of a glacier; but they leave no doubt as to the 
fact of the sliding movement. Again, it is observed that existing glacial valleys, and 
those which are believed to have been such in former times, always indicate, by their 
smoothed and striated rocks, the sliding movements of the glaciers they formerly con- 
tained. In fact, few glacialists at present, I imagine, will doubt the existence of this 
sliding motion, or that it forms a considerable portion of the whole motion of a glacier ; 
and I believe that the experiments above described afibrd an adequate explanation of 
the cause and character of that motion. I insist on this more particularly because the 
explanation has been singularly ignored and misunderstood. The non-applicability of 
the experiments has been asserted, because the sliding mass was not obstructed in its 
motion by lateral obstacles, like a glacier, whereas, in fact, they had no concern with 
lateral obstacles, being merely intended to explain the action of the bed of the valley 
on the superincumbent glacier. The irregularity of the sides introduces, as we shall see 
very shortly, no ditficulty or ambiguity into my views of the subject. I may also state 
that, several years after my experiments and his own observation above stated were pub- 
lished, Principal Foebes repeats his objection of the difiiculty of conceiving the possibi- 
lity of the motion of sliding glaciers being unaccelerated, whereas every one now acknow- 
ledges that they do slide, and knows that their motion is unaccelerated $. M. Agassiz, 
on the contrary, after repeating the experiments, allows that the results remove the 
great difiiculties of admitting the sliding motion of glaciers §. This kind of motion, as 
we have seen, depends very much on the temperature of the lower surface of the glacier 
being always equal to the freezing-temperature. That such must always be the case I 
proceed to show. All observations indicate that it is so; but still, since few direct 
observations can be made on this point, it may be well to show that it follows from 
the temperature of the earth, and the nature and conductivity of ice, that the tempera- 
ture of the lower surface of a glacier must be that above mentioned. 
* Occasional Papers, p. 175. t Glaciers of tBe Alps, p. 289. 
t “ The main objection, however, is this, that a sliding motion of the kind supposed, if it commence must 
he accelerated by gravity, and the glacier must slide from its bed in an avalanche. The small slope of most 
glacier-vaUeys, and the extreme irregularity of their bounding walls, are also great objections to the hypo- 
thesis.” — Occasional Papers ; also published in 1855 in the ‘ Encyclopaedia Britannica.’ 
§ Systeme Glaciaire, p. 568. 
MDCLCLXII. 5 B 
