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The ice of a glacier moves most quickly in the middle, 
and again the surface moves more quickly than the ice below 
the surface. Precisely like a river of a viscous fluid, the sides 
and bottom are retarded by friction against the bed. Again, 
in passing down a sinuous channel, the point of swifter 
motion is not the centre, but always in the outside of the 
curve, the ice following exactly the same law as the current 
of water in a river. When the valley narrows, the ice cur- 
rent becomes more rapid, when the valley widens, the ice 
stream extends its width. Wherever pressure is put on the 
ice it slowly yields, but it does not yield to extension in the 
slightest degree. Hence, wherever the ice of a glacier is 
subjected to tension crevasses open, whose direction is trans- 
verse to the line of stream. 
The motion of a glacier under pressure, is dependent on 
the fact, that the temperature of the glacier throughout its 
mass is little less than the freezing point. Its mass is pene- 
trated by streams of ice-cold water in every direction, and 
its temperature can then be little difierent from 0"" C. The 
only direct observations I have been able to find on the 
internal temperature of a glacier are those of Agassiz, who 
found that in a hole sunk 200 feet deep in the ice a thermo- 
meter averaged 31*24° F., and in winter 28-24 ° 
The daily motion of a glacier in winter is in fact found to 
be about half of its summer motion. 
If the temperature of a glacier is 0° C, the slightest 
pressure is sufficient to cause it to melt at the point stressed, 
but if its temperature be below 0"" C, the pressure must reach 
a certain amount, which can be calculated, before it can move, 
so that in a cold climate the ice will be actually a rigid body, 
and will not move under the same condition as in a warmer 
climate. In fact, it can only move by the process of crushing. 
If the temperature of a glacier were l"" C. below freezing, 
it would require a pressure represented by a column of ice 
