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INTERNAL STRUCTURE AND 
vertical pressure, must bo added the pressure 
from behind, which is most effective where the 
mass is largest and the amount of motion in a 
given time greatest. In the glacier, the mass 
is, of course, largest in the centre, where the 
trough which holds it is deepest, and least on 
the margins, where the trough slopes upward 
and becomes more shallow. Consequently, the 
middle of a glacier always advances more rapidly 
than the sides. 
Were the slope of the ground over which it 
passes, combined with the pressure to which the 
mass is subjected, the whole secret of the onward 
progress of a glacier, it is evident that the rate 
of advance would be gradually accelerated, reach- 
ing its maximum at its lower extremity, and 
losing its impetus by degrees on the higher levels, 
nearer the point where the descent begins. This, 
however, is not the case. The glacier of the 
Aar, for instance, is about ten miles in length ; 
its rate of annual motion is greatest near the 
point of junction of the two great branches by 
which it is formed, diminishing farther down, 
and reaching a minimum at its lower extremity. 
But in the upper regions, near their origin, the 
progress of these branches is again gradually 
less. 
Let us see whether the next cause of displace- 
ment, the infiltration of moisture, may not in 
