ME. HOPKINS ON THE THEORY OE THE MOTION OF GLACIERS. 
743 
been destroyed by the previous crushing; and then the angular distortion, the force F, 
and the increase of the maximum pressure are reproduced, and the mass is crushed again. 
We thus understand how the alternate processes of crushing and regelation must be 
repeated when the mass is in motion, though the weight alone of the mass, however 
great, might not, and probably would not, if unaided by the distortion arising from the 
motion, be sufficient to crush any even the lowest portions of the glacier, any more than 
the superincumbent weight of a mountain crushes the lower portion of the strata which 
compose it. We have seen how this power of distortion is increased by the sliding of 
the glacier. 
It is thus that I conceive the process of regelation in glaciers to be intimately united 
with their uniform sliding movement. Both these properties are proved to belong to ice 
at the same particular temperature (that of freezing), by clear and conclusive experi- 
ments, and form, as I have endeavoured to show, the foundation of a theory which 
explains all the principal observed phenomena in the motion of glaciers, without assign- 
ing to ice any property, like viscosity accurately defined, which it cannot be experiment- 
ally proved to possess. In the introduction to this paper I have shown that the pro- 
perty of regelation is totally distinct from that of plasticity or viscosity, if by either of 
those terms it is intended to designate a determinate property of bodies distinct from 
that of solidity. All the experimental elucidations which have been given of the 
Viscous Theory have been drawn from bodies, such as moist plaster of Paris, tar, treacle, 
unconsolidated lava, «&c., all of which may with strict propriety be termed plastic or 
viscous ; but I maintain that ice possesses no property which at all assimilates it to those 
substances. I have shown that the ajpjparent plasticity of ice, as manifested in the com- 
pressions of glaciers under great pressure in the course of their motion, is no determinate 
property of ice at all, but a consequence of its fracture and regelation. Still, differing as 
I do from the author of the Viscous Theory, I consider the scientific world largely 
indebted to him for his unwearied researches respecting glacial phenomena, for the large 
amount of general and detailed knowledge which he has communicated to us, and the 
interest with which he has helped to invest the subject. Moreover, though he may not 
have been the first to observe that fundamental character in the motion of a glacier 
which consists in the more rapid motion of its axial portions, he was the first glacialist 
whose mind was thoroughly imbued with the necessity of recognizing the influence of 
molecular mobility in the mass of a glacier on its general onward motion. In like 
manner, though not absolutely the first to observe that interesting and characteristic 
structure of glacial ice, the veined structure, he was doubtless the first to recognize the 
existence of law in that structure, and its consequent importance as indicating the opera- 
tion of causes physical or mechanical, or both, with which we were entirely unacquainted. 
In all this. Principal Foebes has contributed largely to the sound progress of glacial 
science, and his labours must always be highly appreciated. Beyond this boundary, he 
stepped into the region of speculative theory ; and it appears to me, as may be seen from 
much I have said in this paper, that he did so without any distinct definition of the 
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