ME. HOPKINS ON THE THEOEY OF THE MOTION OF HLACIEES. 
739 
Again, in any actual case of a fluid sustained in a capillary tube, the inner surface of 
the tube will not generally be a surface of free equilibrium like the upper surface of the 
fluid itself; and therefore there must necessarily be some action between the tube and 
fluid, should they remain in contact, even if there were no attraction, like that just con- 
sidered, between them. Now it can be shown, according to the fundamental principles 
on which the theory of capillarity is founded, that in all cases resembling the one we 
have been considering, the action in question must be equivalent to a tension on the 
inner surface of the tube. Hence this action, as well as that due to gravity, produces 
forces on the tube which tend to contract and not to expand it, while the mutual attrac- 
tions of the tube and fluid produce directly neither the one tendency nor the other. 
Consequently the general tendency of capillary action in the interior of a glacier will 
be to contract and not to expand its dimensions. The contrary notion has probably 
arisen from the idea that gravity would produce a pressure on the interior surface of a 
capillary tube like that which it produces in a tube of larger dimensions. The fluid 
which may exist in these capillary tubes will, of course, add its own weight to that of 
the glacier, an addition probably too insignificant to produce the slightest sensible 
influence. 
65. There is something so plausible at first sight in the idea of a great internal fluid 
pressure within a glacier, due to the water percolating through it, or suspended within it 
by capillarity, while additional weight has been given to this notion by the advocacy of 
the author of the Viscous Theory, that I have thought it essential to enter into more 
details on the subject than I should otherwise have deemed necessary, in order to 
prove the fallacy of the conclusions which have been deduced from erroneous views 
respecting it. 
I now proceed to explain how the intensity of the internal forces is increased by the 
sliding movement of the glacier. 
66. Effect of the Sliding of a Glacier on the Internal Pressures and Tensions. — It has 
frequently been objected to the sliding of a glacier, that the inequalities along the sides 
of the containing valley would eflectually prevent such motion. This objection, how- 
ever, will be entirely obviated, if we eliminate the uncertain and irregular opposing 
forces arising from local lateral obstacles, by substituting for the sides of the glacial 
valley two imaginary vertical planes near and approximately parallel to them, so that 
we shall thus have the tangential action of ice along these planes as the retarding force 
on the general mass of the glacier, instead of the action of the walls of the valley. The 
greatest magnitude of this retarding force must be equal to the greatest tangential cohe- 
sive power of the ice. This is, in fact, the greatest force which the walls, however irre- 
gular, could possibly exercise in opposing the sliding past them of the general mass of 
the glacier. To simplify our problem as much as possible, I shall also first suppose the 
glacier of uniform width and thickness, and the inclination of its bed to be likewise 
uniform. We may then consider A and B each =0. I shall also suppose C=0, and 
