Change>i in the Muir Glacier. — Baldwin. 373 
-canon and Main valley to the ice-front, although the upper por- 
tions now move back toward their parent ledges. 
The most noticeable change was the recession of the ice-front 
three thousand feet during the four years from 1886 to 1890. 
The position of the ice-front depends mainly on the rate of motion 
•of the ice and on the rate at which the front melts back or breaks 
off. The aerial melting is evidently a small factor in the retreat 
of the water front, and conditions under water are subject to but 
very slight variations, as of temperature or tides. It follows, there- 
fore, that this recession can only be due to reduced rate of motion. 
In 1886 the ice-front was from 250 to 300 feet high above the 
water, and the general surface where the measurements were taken 
was a little over 400 feet above the water. In 1890 the hight of 
ice-front varied from 126 feet to 210 feet,* averaging 170 feet. 
The difference is much greater than at first appears, for this level 
of 170 feet was back at the point where Prof, Wright's furthest 
measurements were taken, that is, where the ice stood over four 
hundred feet above the water in 1886. Since the slope of the 
glacier from the front was nearly the same in both periods, we 
must conclude that the entire surface of the glacier was two hun- 
dred and thirty feet higher in 1886. 
The moving force of the glacier is that part which is above the 
sea level, for it is evident that the ice below sea level is i-estrained 
from originating any tendency to motion outward by the presence 
of the waters of the bay. This moving force, then, in 1886, was 
two and one-third times as great as in 1890, or, if we drop one- 
third to allow for error, it was at least twice as great, while the 
mass to be moved, if we suppose the glacier to be about one 
thousand feet thick, was only about one-fifth greater. Just what 
the effect may be of such a difference in vertical front, is difficult 
to sa}', but it is certain that any cause affecting the entire surface 
of the glacier must have a much intensified effect, when its results 
are condensed into the small space allowed by the narrow opening 
into Muir Inlet. 
B. On account of the general low condition and the position 
of the ice-front in 1890, the western branch seems to have added 
little, if any, pressure toward the outlet, while in 1886 it pushed 
directly toward and into the outlet. 
C. The glacier in 1890 was discharged through an outlet fifteen 
*See map by Prof. Reid, Nat. Geog. Mag., vol. iv., p. 55. 
