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ME. HOPKINS ON THE THEOEY OE THE MOTION OP GLACIEES. 
me either to demand refutation as erroneous, or admission into any well-founded theory 
as correct. This treatment of the subject must necessarily lead to a certain repeti- 
tion as to results already established, and be also of too critical a character, perhaps, 
with reference to other results in which I may have no confidence. I can only say that, 
in the present state of glacial theory, such defects must be inherent in any attempt to 
present it under a more complete and systematic form than it has hitherto, I think, 
assumed. It is this circumstance, too, which I would especially ofier as an apology for 
the repetition of certain results which I obtained many years ago. Most of them are 
abstract mathematical results. They are here obtained by a more general analysis of 
the problem than that formerly employed, and are introduced as essential steps in the 
development now offered of the theory of the motion of glaciers. 
In the first section I shall endeavour to remove the ambiguities which have beset this 
subject from the want of explicit definitions of certain terms expressing properties of ice 
on which our theories of glacial motion must essentially depend. 
In the second section I shall give a brief statement of the results of experiments 
which explain the sliding of a mass of ice down a plane of small inclination, with a 
slow and unaccelerated motion like that of an actual glacier. 
In the next section certain propositions are investigated respecting the internal pres- 
sures and tensions superinduced within a solid body by external forces which slightly 
distort it, and produce a small relative displacement of its component particles, or what 
may be termed a molecular disjplacement. 
In the subsequent sections these results are applied to the explanation of crevasses, 
and to an examination of the theories which have been proposed of the veined structure 
of glacial ice. Finally, I have shown the importance of the sliding motion in ginng 
efficiency to the internal pressures and tensions to dislocate the glacier, which thus 
becomes relieved from its internal strain, regaining the continuity of its mass and structure 
by its property of regelation so beautifully exhibited in Dr. Tyndall’s experiment. 
Section I. — Definitions and Explanations of Terms. 
I. The external forms of all bodies in nature may be changed in a greater or less 
degree, and without producing discontinuity in their mass or destruction of their internal 
structure, by the action of any external forces, the original or undisturbed form from 
which the change of form is to be estimated being that which the body would assume 
if acted on by no external forces whatever. This change of form necessarily implies a 
change in the relative positions of the component particles of the mass, or a certain 
greater or smaller amount of molecular mobility., or power in the particles of moving 
inter se. We may speak either of the general change of the form of the whole body, or 
of that which takes place in each of its small elementary portions ; it is, in fact, in this 
latter sense that we are obliged to regard it in any accurate investigations, because the 
change of form for different elements will usually be different. Change of form in an 
element may or may not be accompanied by a change of its volume. In the first case it 
