44 
CAUSES OF GLACIER MOTION. 
difference of temperature of only twenty degrees between 
mid-day and mid-night there is as much as 20ft. change of 
length in the glacier daily, lengthening 20ft. every day and 
shortening again nearly, but not quite, 20ft. every night. 
In the case of the Greenland glacier, which moves at the 
extraordinarily rapid rate of twenty-two times as fast as the 
Swiss glaciers, there is an enormously greater moving power 
if, as there seems good reason to assume, the entire surface of 
that lame countrv, 700 miles in width and 1400 miles in 
length, is covered with a thick mass of snow, consolidated 
more or less throughout the whole extent to the condition of 
glacier ice. Then the whole of that immense area will 
expand daily from the centre on all sides towards the coast, 
and the only outlet for this expansion will be along the 
several valleys and ravines leading to the sea, producing a 
glacier along each of them. The distance from the centre of 
the country to the coast, 350 miles, gives a daily expansion 
of no less, than 50ft. per degree of temperature, or a total of 
1000ft. for the twenty degrees difference of temperature taken 
before, as occurring daily between mid-day and mid-niglit. 
The result of this would be a daily movement of advance and 
receding of 1000ft. all round the coast if there were a free 
exit all the way round ; and when the exit is confined to the 
several valleys and ravines leading to the sea, through which 
the whole advancing mass of ice has to be crowded, it does 
not seem unreasonable that the actually observed result 
should be obtained, namely, an actual advance of 65ft. per 
day in the coast glacier. 
When it is considered that this enormous propelling force 
is constantly forcing outwards the glaciers through the only 
outlets by which they can escape, and that this escape of the 
ice must take place, whether the glacier bed is at a great or a 
small inclination, though not at the same rate, it will be seen 
that a glacier may exist with a very small slope, such as the 
slope 1 in 120 named of the Greenland glacier, and 
that if the bed were to become level, or even turn 
upwards towards the outlet, the glacier motion could not 
be stopped. This latter case actually exists, and there is a 
glacier that really moves up hill in the latter part of its 
course, a circumstance that does not seem to admit of 
explanation by any other theory than the one described. 
The transverse crevasses of glaciers are a marked feature; 
they are numerous irregular cracks of large dimensions, 
extending a great depth into the mass of ice, and have been 
suggested to be caused by a bending of the ice stream over 
the edge of an increased slope in the bed of the glacier. But 
