202 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
express, no mathematical symbols weave into a demonstration. I can only say that 
it is easier to believe than to disbelieve ; and that sooner or later, it will, I doubt not, 
be generally admitted. 
Considering the crevasses as chiefly superficial in the normal glacier (I mean that 
of which the inclination is not excessive), it is evident that the formation of the cre- 
vasses must depend mainly upon the configuration of the bed. Where the section of 
the bed parallel to the length of the glacier is convex upwards, there the tension at 
the surface will cause the crevasses to expand; when the bed is concave and the 
surface is being compressed, the crevasses tend to close. Hence the surface of the 
glacier descending an irregular bed may be alternately in a state of distension and 
compression, and the crevasses do not tend to widen indefinitely, which would be the 
case if the whole glacier were distended. This tendency in the crevasses to expand 
and contract in accordance with their position is beautifully seen in viewing the Mer 
de Glace from a height, as we have recommended. The steep fall opposite Trelaporte 
shows the expansion of the crevasses, but the comparative level opposite the little 
glacier of Charmoz gives it time to recover its solidity by the general closing of the 
crevasses under compression. The careful study of such a scene as this gives a more 
clear insight into the glacier phenomena than any other part of the inquiry, except- 
ing only the measurement of velocities. 
Law of Velocities . — To these velocities we now return. The varying velocities in 
different glaciers, at different seasons and in different parts of the same season, are 
all in accordance with the motions of a viscous or plastic body. They depend upon 
the slope, being greatest, ceteris paribus , when the slope is greatest ; and upon the 
climate to which the glacier is exposed, being greatest in glaciers which descend into 
deep valleys, and least in those which, though very steep (such as that of the Schon- 
horn described in § 6), are placed in so elevated and therefore dry and cold an 
atmosphere as to afford in sufficient water to moisten the snowy mass or ntfve, and 
which are therefore endowed with very feebly hydrostatic qualities. This is demon- 
strated on the one hand by the extreme smallness of their motions, and on the other 
by the insignificant streams of water to which they give birth even in the height of 
summer. In any individual glacier the velocity of the parts must (on any theory) 
vary with the area of section through which the ice stream has to pass; but yet it 
may happen that the contraction of a valley, if not accompanied (as is often the case) 
with an increased slope, will oppose so great a resistance to the efflux of the mass, 
that under intense longitudinal compression its forward motion is retarded, and the 
condition of uniform discharge is satisfied by the accumulation of the ice in a vertical 
direction, the rise of the surface being necessarily accompanied with a thrust from 
below upwards, and a sliding of the particles over one another in that direction. 
This appears conclusively to be the case for a great extent of the lower part of the 
glacier of the Aar, as already mentioned, and affords the most direct evidence which 
