i88o.] 
On Water and Air . 
321 
its motion augments from the banks to the centre, because 
at the centre it is free from the friction of the banks. Take 
the case of a river which flows through a sinuous valley. 
Suppose it to be coming down with considerable power, and 
having to make a bend. The most wondrous example of 
that kind in the world is perhaps the so-called whirlpool 
below the Falls of Niagara. There the River Niagara takes 
a sudden bend at right angles to its previous direction. The 
water comes down plunging with impetuous force, and im- 
pinges against the bank on the other side. It is thus caused 
to turn round and produce the wondrous whirlpool from 
which the water escapes on to Lake Ontario. But it is no 
matter how small the bend may be. The least bending, 
such as we see here exhibited in the Mer de Glace, has this 
effect in the case of a river. When the water moves down- 
wards it is always carried on towards the concave bank of 
the river; and if the river bends in the other direction the 
greatest motion of the river will not follow the middle line, 
but will change in accordance with the flexures of the valley 
through which the river passes. This is the law of the 
motion of water ; the law of the motion of ice is precisely 
similar. For instance, the continuous line x Y in the diagram 
(Fig. 31) represents the centre of the glacier. The dotted 
line marks the maximum motion of the glacier ; so you see 
that the swiftest motion is, as I have said, towards the con- 
cave side of the glacier. At Trelaporte the swiftest point 
is towards e on the line E e'. Then you have a point where 
the greatest motion coincides exactly with the middle of the 
glacier ; then we come lower down, and here the point of 
greatest motion passes to the other side of the centre, and 
is towards D on the line d d' ; and then afterwards it passes 
back again across the centre of the glacier, and its greatest 
motion is at c on the line c c'. Thus, as I have said, the 
motion of the glacier downwards resembles in every parti- 
cular the motion of a river. 
Let us turn again to our map (Fig. 30), and mark the 
wonderful power of the ice to mould itself to the valley 
through which it presses. See the extraordinary behaviour 
of the glacier in coming down that valley; and how the ice 
can accommodate itself to the flexures of the valley ; and 
how the immense masses of ice which are tributaries of the 
Mer de Glace (the Cascade du Talefre, the Glacier du Geant, 
the Glacier de Lechaud) weld together, and squeeze them- 
selves into the extraordinary small space we find at the 
gorge at Trelaporte. Now it is possible for ice to be squeezed 
in this way, changing its form but not its volume, and 
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