ME. HOPEINS ON THE THEOET OF THE MOTION OF GLACIEES. 
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portions of the glacier, where the superincumbent weight can have no very sensible 
influence. These questions may be partly answered by a careful consideration of the 
theoretical explanations which have been given in the preceding pages ; but for their 
complete answer they require, I conceive, the recognition of a kind of motion which has 
been till recently so entirely neglected. I allude to the motion by which a glacier slides 
over the bed on which it reposes (Sect. II. art. 7). I shall shortly proceed to explain 
the influence of this motion on the intensities of the internal pressures and tensions. 
Piincipal Foebes was struck with this difficulty respecting the adequacy of the internal 
forces to produce the crushing effects ascribed to them, and the consequent mobility of 
the component particles inter se, though, regarding ice as viscous, the difficulty may 
naturally be supposed to have appeared less to him than if he had regarded it as a solid 
substance. He puts the difficulty in the following form*; — “ Were a glacier composed of 
a sohd crystalline cake, fitted or moulded to the mountain bed which it occupies like a 
lake tranquilly frozen, it would seem impossible to admit such a flexibility or yielding 
of parts as should permit any comparison to a fluid or semifluid body transmitting 
pressure horizontally, and whose parts might change their mutual position, so that one 
part should be pushed out whilst another remained behind.” The difficulty as thus 
stated is equivalent to that above mentioned. Principal Foebes meets it as follows : — 
“ But we know in point of fact that a glacier is a body very differently constituted. It 
is clearly proved by the experiments of Agassiz and others, that a glacier is not a mass 
of ice, but of ice and water, the latter percolating freely through the crevices of the 
former to all depths of the glacier ; and as it is a matter of ocular demonstration that 
these crevices, though very minute, communicate freely with one another to great 
distances, the water with which they are filled communicates force also to great distances, 
and exercises a tremendous hydrostatic pressure to move onwards, in the direction in 
which gravity urges it, the vast porous crackling mass of seemingly rigid ice in which it 
is, as it were, bound up. 
“Now the water in the crevices does not constitute the glacier, but only the principal 
vehicle of the force which acts on it ; and the slow irresistible energy with which the icy 
mass moves onwards from hour to hour with a continuous march, bespeaks of itself a 
fluid pressure. But if the ice were not in some degree ductile or plastic, this pressure 
could never produce any, the least, forward motion of the mass. The pressures on the 
capillaries of the glacier can only tend to separate one particle from another, and thus 
produce tensions and compressions, within the hody of the glacier itself which yields, 
owing to its slightly ductile nature, in the direction of least resistance, retaining its con- 
tinuity, or recovering it by reattachment after its parts have suffered a bruise, according 
to the violence of the action to which it has been exposed.” The author again remarks 
(p. 167), “ If it were not for the capillarity of the ducts, it is plain that no effective 
hydrostatic pressure could be developed at all ; the flow being equal to the supply, no 
part of the vis viva would be expended in producing internal pressures.” 
• Occasional Papers, p. 165. See also a memoir in the Transactions of the Eoyal Society for 1816, Part III. 
