JIE. HOPKINS ON THE THEOEY OE THE MOTION OF GLACIERS. 
683 
removed a great stumbling-block in glacial theory. In fact, the viscous theory assigns 
no physical cause for the reunion in question. All we could do, before the publication 
of those experiments, was to infer from the observed facts that ice did possess some 
property which facilitated the reunion of separate pieces in contact ; but this Avas like 
the attempt to define viscosity by an appeal to the phenomena which that property was 
intended to explain. 
6. An imperfect plasticity in ice has sometimes been spoken of The fact is, all solid 
bodies might be said to have an imperfect plasticity, if we chose to admit this vague- 
ness in scientific language, since all are capable of greater or less extension or compres- 
sion. As to the apparent plasticity inferred from the motion of glacial masses, and 
arising from the crevicing of the ice, I have explained that it has no relation whatever 
to real plasticity. Such crevices are the necessary consequences of the external forces 
acting on the glacier, and are as essential to the theory of regelation as they are uncon- 
nected with any property of plasticity. 
I proceed, as proposed in my introductory remarks, to explain the sliding of glaciers. 
Section II. — Sliding Motion of Glaciers along the bottoms of the Valleys containing them. 
7. The sliding motion of glaciers, as first suggested by De Saussuee, seemed to involve 
some serious difficulties. The inclination of the surfaces along which some of the 
Alpine glaciers descend is so small (not exceeding, perhaps, in some cases 3° or 4°) as 
to furnish one very obvious objection ; and another was, that if glaciers did thus move 
at all, it must be by an accelerated motion, whereas their real motion was an unac- 
celerated one. Both these objections were founded on an entirely erroneous conception 
of the nature of the forces called into action in the sliding of a glacier. They were 
considered to be analogous to the force of friction in the ordinary case of a body sliding 
down an inclined plane. But in this latter case the constitution of the sliding body, and 
of that on the surface of Avhich it slides, is always assumed to be such that the surfaces in 
contact are not affected by the sliding movement, and then Ave haA'e the experimental 
law that the friction is independent of the velocity. Consequently the motion is an 
accelerated one. But the condition just mentioned, respecting the surfaces in contact, 
Avill not be satisfied in the case of a glacier, assuming Avhat I shall prove shortly, that its 
lower surface must necessarily have a temperature not lower than that of freezing, and 
must consequently be always on the point of disintegration by thawing. Ice then 
becomes very tender, and the cohesion of its particles at its lower surface becomes 
insufficient to prevent the descent of a mass of such an enormous Aveight as that of a 
glacier, even along planes of the smallest inclination. This was clearly illustrated by a 
simple experiment Avhich I made some years ago, which also fully explained the unacce- 
lerated motion of a sliding glacier. The details of this experiment Avill be found in the 
Cambridge Transactions for 1844^. It constitutes so fundamental a step in this subject, 
according to my own views of it, that I Avould beg permission to give here a brief descrip- 
tion of it, and of the results to which it leads. 
* They will also be found in the Philosophical Magazine for January 1845, 
