"(2) For small pressures, and up to a certain well defined limit
of pressure, the coefficient is fairly large, having the value
0.36±.01 in the case investigated.
"(3) For pressures greater than the above limit the coefficient
is relatively small, having the value 0.17±.01 in the case
investigated."
It will be seen that Morphy's results are similar to those
arrived at in the first experimental consideration of our
subject; but from the manner in which the experiments have been
carried out, they are more accurate and reliable.
A great deal more might be said about skating, and the allied
sports of tobogganing, sleighing, curling, ice yachting, and
last, but by no means least, sliding—that unpretentious pastime
of the million. Happy the boy who has nails in his boots when
Jack-Frost appears in his white garment, and congeals the
neighbouring pond. But I must turn away at the threshold of the
humorous aspect of my subject (for the victim of the street
"slide" owes his injured dignity to the abstruse laws we have
been discussing) and pass to other and graver subjects intimately
connected with skating.
James Thomson pointed out that if we apply compressional stress
to an ice crystal contained in a vessel
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which also contains other ice crystals, and water at 0° C., then
the stressed crystal will melt and become water, but its
counterpart or equivalent quantity of ice will reappear elsewhere
in the vessel. This is, obviously, but a deduction from the
principles we have been examining. The phenomenon is commonly
called "regelation." I have already made the usual regelation
experiment before you when I compressed broken ice in this mould.
The result was a clear, hard and almost flawless lens of ice. Now
in this operation we must figure to ourselves the pieces of ice
when pressed against one another melting away where compressed,
and the water produced escaping into the spaces between the
fragments, and there solidifying in virtue of its temperature
being below the freezing point of unstressed water. The final
result is the uniform lens of ice. The same process goes on in a
less perfect manner when you make—or shall I better say—when you
made snowballs.
We now come to theories of glacier motion; of which there are
two. The one refers it mainly to regelation; the other to a real
viscosity of the ice.
The late J. C. M'Connel established the fact that ice possesses
viscosity; that is, it will slowly yield and change its shape
under long continued stresses. His observations, indeed, raise a
difficulty in applying this viscosity to explain glacier motion,
for he showed that an ice crystal is only viscous in a certain
structural
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