CAUSES OF GLACIER MOTION. 
43 
the conditions that are observed. Ice lias been looked upon 
as a “ viscous” body, and having something of the properties 
of pitch, which, although brittle and fracturing like ice when 
cold, will, if a lump is left for some time, gradually sink down 
and flow outwards on all sides, or travel down an inclined 
base, the rapidity of motion depending upon the temperature 
to which it is exposed. But this theory is found to be not 
applicable to glaciers, because their motion is found to be 
independent of temperature to a great extent, and their rate 
of travel is nearly as great in winter as in summer, being 
about four-fifths as much in winter as in summer. 
The only explanation that appears to fit all these circum¬ 
stances is the one originally proposed by the late Canon 
Moseley, namely, that the motion is caused by the daily 
alteration of expansion and contraction in the ice from the 
exposure to the direct heat of the sun in the day and the 
intense cold of the night. The amount of this action will 
depend not on the actual temperature at any time, but upon 
the difference of temperature between mid-day and mid-niglit, 
which will not be much less in amount in winter than in 
summer. When the expansion of the ice takes place in the 
glacier it will have effect mainly in the direction where move¬ 
ment is easiest, that is, down the valley ; but when the con¬ 
traction subsequently takes place, as the expansion is assisted 
by gravity whilst the contraction is opposed by gravity, the 
movement upwards from contraction will be somewhat less in 
amount than the movement downwards from expansion, and 
the result will be that the mass will be permanently displaced 
a small distance down the valley. By the repetition of such 
alternate daily expansions and contractions the glacier is 
moved gradually from the top to the bottom of its course. 
This action is directly analogous to what is well known as 
the creep in the lead covering of roofs; the lead is constantly 
endeavouring to creep down the slope of the roof, in conse¬ 
quence of being exposed to daily alternations of expansion and 
contraction from change of temperature, and any movement 
downwards is assisted by gravity, whilst the return movement 
upwards is opposed by gravity. That the total force available 
in this manner in the case of a glacier is very large in amount 
will be seen from a consideration of the circumstances. It 
happens that the rate of expansion and contraction of ice 
under changes of temperature is very high, being as much as 
1—35,000th of the length foi-eacli degree of temperature Falir. 
(which is three times as great as the rate of expansion in 
brass); and in the case of the Elione Glacier, six miles in 
length, this amounts to about 1ft, for each degree, or for a 
