204 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
“ that the main cause of the restoration of the surface is the diminished fluidity of 
the glacier in cold weather, which retards (as we know) the motion of all its parts, 
but especially of those parts which move most rapidly in summer. The dispropor- 
tion of velocity throughout the length and breadth of the glacier is therefore less ; 
the ice more pressed together and less drawn asunder ; the crevasses are consolidated, 
while the increased friction and viscosity causes the whole to swell, and especially 
the inferior parts, which are most wasted*.” I have nothing to add to this explana- 
tion, except that the observation of the motion throughout the whole year confirms 
it in every particular. The more elevated portions of the glacier, which during a 
large portion of the year are exposed to a mean temperature under 32°, move in a 
manner comparatively uniform, the lower extremities undergo great oscillations in 
their speed (in the ratio of four or five to one ; see page 185) ; hence the atte- 
nuation during the summer regime which is owing to the drag taking place down- 
wards in an excessive degree ; but the winter’s cold, equalizing in some measure the 
velocity everywhere, brings the plasticity into full action, fills the crevasses and 
swells the surface to its old level. 
As it is universally admitted that the glacier proper does not grow in thick- 
ness by snowy accumulations, the important variations in its level in different years^ 
cannot be ascribed to the severity of certain seasons increasing the mass of snow 
falling upon it, but rather to the prolongation of the winter cold into spring and 
summer, which causes the condensing or accumulating process to be in excess, and 
therefore the thickness of the plastic mass to accumulate beyond its due amount. 
Thus we have the following phenomena, all independently observed, reconciled 
and explained by one hypothesis ; the general convexity of the crevasses upwards, 
notwithstanding the excess of motion in the centre ; the general verticality of the 
crevasses, notwithstanding the retardation of the bottom ; the perfect state of the 
crevasses every spring succeeding their visible collapse in autumn; the ascertained 
velocity of different parts of the glacier, and the diversity of the annual changes which 
these velocities present; the seemingly opposed facts showing the glacier to be sub- 
jected to powerful tension, producing crevasses, and yet to be under a compression 
which produces in some places the frontal dip ; and finally, the renewal of the level 
of the ice during winter, which has been lost partly by superficial melting, but as 
much or more so by the attenuation and collapse of the glacier during summer. 
These various effects of one cause, though they do not embrace all the phenomena of 
glaciers, certainly include a very remarkable and complicated group of facts. 
* Travels, p. 386, 2nd edit. 
f For instance, it has been seen from Balmat’s narrative (p. 189 above), that in 1845 the glacier attained a 
much higher level at the Angle than it had done for three previous years at least, since all the marks of mea- 
surements which were cut on the rock in 1842 were concealed : and he attributes this, apparently with reason, 
to the extreme lateness and coldness of the spring. 
