PROGRESSION OF GLACIERS. 
267 
season at which the temperature of the glacier 
sinks below 32° Fahrenheit. The simple fact, 
that in the spring the glacier swells on an aver- 
age to about five feet more than its usual level, 
shows how important this infiltration must be. 
I can therefore only wonder that other glacialists 
have given so little weight to this fact. It is ad- 
mitted by all, that the waste of a glacier at its 
surface, in consequence of evaporation and melt- 
ing, amounts tc about nine or ten feet in a year. 
At this rate of diminution, a glacier, even one 
thousand feet in thickness, could not advance 
during a single century without being exhausted. 
The water supplied by infiltration no doubt re- 
pairs the loss to a great degree. Indeed, the 
lower part of the glacier must be chiefly main- 
tained from this source, since the annual increase 
from the fresh accumulations of snow is felt only 
above the snow-line, below which the yearly snow 
melts away and disappears. In a complete the- 
ory of the glaciers, the effect of so great an ac- 
cession of plastic material cannot be overlooked. 
I now come to some points in the structure of 
the glacier, the consideration of which is likely 
to have a decided influence in settling the con- 
flicting views respecting their motion. The ex- 
periments of Faraday concerning regelation, and 
the application of the facts made known by the 
great English physicist to the theory of the gla- 
