PROGRESSION OF GLACIERS. 
237 
when the water formed during the day at its 
surface and rushing over it in myriad rills had 
ceased to flow. 
While we admit a number of causes as af- 
fecting the motion of a glacier, — namely, the 
natural tendency of heavy bodies to slide down 
a sloping surface, the pressure to which the mass 
is subjected forcing it onward, the infiltration of 
moisture, its freezing and consequent expansion, 
— we must also remember that these various 
causes, by which the accumulated masses of snow 
and ice are brought down from higher to lower 
levels, are not all acting at all times with the 
same intensity, nor is their action always the 
same at every point of the moving mass. While 
the bulk of snow and ice moves from higher to 
lower levels, the whole mass of the snow, in 
consequence of its own downward tendency, is also 
under a strong vertical pressure, arising from its 
own incumbent weight, and that pressure is, of 
course, greater at its bottom than at its centre 
or surface. It is therefore plain, that, inasmuch 
as the snow can be compressed by its own weight, 
it will be more compact at the bottom of such 
an accumulation than at its surface, this cause 
acting most powerfully at the upper part of a 
glacier, where the snow has not yet been trans- 
formed into a more solid icy mass. To these 
two agencies, the downward tendency and the 
