PROGRESSION OF GLACIERS. 
265 
tract anew, their surface wasting so little from 
external influences that they advance far below 
the line of perpetual snow without any sensible 
diminution in size, it is evident that an enormous 
pressure must have been brought to bear upon 
them before they could have been packed into 
the lower valleys through which they descend. 
Physicists seem now to agree that pressure is 
the chief agency in the motion of glaciers. No 
doubt, all the facts point that way ; but it now 
becomes a matter of philosophical interest to de- 
termine in what direction it acts most powerfully, 
and upon this point glacialists are by no means 
agreed. The latest conclusion seems to be, that 
the weight of the advancing mass is itself the 
efficient cause of the motion. But while this is 
probably true in the main, other elements tend- 
ing to the same result, and generally overlooked 
by investigators, ought to be taken into considera- 
tion ; and before leaving the subject, I would add 
a few words upon infiltration in this connection. 
The weight of the glacier, as a whole, is about 
the same all the year round. If, therefore, press- 
ure, resulting from that weight, be the all-con- 
trolling agency, its progress should be uniform 
during the whole year, or even greatest in win- 
ter, which is by no means the case. By a series 
of experiments, I have ascertained that the on- 
ward movement, whatever be its annual average, 
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