direction. A complex mixture of crystals such, as we know glacier
ice to be, ought, we would imagine, to display a nett or
resultant rigidity. A mass of glacier ice when distorted by
application of a force must, however, undergo precisely the
transformations which took place in forming the lens from the
fragments of ice. In fact, regelation will confer upon it all the
appearance of viscosity.
Let us picture to ourselves a glacier pressing its enormous mass
down a Swiss valley. At any point suppose it to be hindered in
its downward path by a rocky obstacle. At that point the ice
turns to water just as it does beneath the skate. The cold water
escapes and solidifies elsewhere. But note this, only where there
is freedom from pressure. In escaping, it carries away its latent
heat of liquefaction, and this we must assume, is lost to the
region of ice lately under pressure. This region will, however,
again warm up by conduction of heat from the surrounding ice, or
by the circulation of water from the suxface. Meanwhile, the
pressure at that point has been relieved. The mechanical
resistance is transferred elsewhere. At this new point there is
again melting and relief of pressure. In this manner the glacier
may be supposed to move down. There is continual flux of
conducted heat and converted latent heat, hither and thither, to
and from the points of resistance. The final motion of the whole
mass is necessarily slow; a few feet in the day or, in winter,
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even only a few inches. And as we might expect, perfect silence
attends the downward slipping of the gigantic mass. The motion
is, I believe, sufficiently explained as a skating motion. The
skate is, however, fixed, the ice moves. The great Aletsch
Glacier collects its snows among the highest summits of the
Oberland. Thence, the consolidated ice makes its way into the
Rhone Valley, travelling a distance of some 20 miles. The ice now
melting into the youthful Rhone fell upon the Monch, the Jungfrau
or the Eiger in the days when Elizabeth ruled in England and
Shakespeare lived.
The ice-fall is a common sight on the glacier. In great lumps and
broken pinnacles it topples over some rocky obstacle and falls
shattered on to the glacier below. But a little further down the
wound is healed again, and regelation has restored the smooth
surface of the glacier. All such phenomena are explained on James
Thomson's exposition of the behaviour of a substance which
expands on passing from the liquid to the solid state.
We thus have arrived at very far-reaching considerations arising
out of skating and its science. The tendency for snow to
accumulate on the highest regions of the Earth depends on
principles which we cannot stop to consider. We know it collects
above a certain level even at the Equator. We may consider, then,
that but for the operation of the laws which James Thomson
brought to light, and which his illustrious brother,
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Lord Kelvin, made manifest, the uplands of the Earth could not
have freed themselves of the burthen of ice. The geological
history of the Earth must have been profoundly modified. The
higher levels must have been depressed; the general level of the
ocean relatively to the land thereby raised, and, it is even
possible, that such a mean level might have been attained as
would result in general submergence.
During the last great glacial period, we may say the fate of the
world hung on the operation of those laws which have concerned us
throughout this lecture. It is believed the ice was piled up to a
height of some 6,000 feet over the region of Scandinavia. Under
the influence of the pressure and fusion at points of resistance,
the accumulation was stayed, and it flowed southwards the
accumulation was stayed, and it flowed southwards over Northern
Europe. The Highlands of Scotland were covered with, perhaps,
three or four thousand feet of ice. Ireland was covered from
north to south, and mighty ice-bergs floated from our western and
southern shores.
The transported or erratic stones, often of great size, which are
found in many parts of Ireland, are records of these long past
events: events which happened before Man, as a rational being,
appeared upon the Earth.