PROGRESSION OF GLACIERS. 
251 
at a rate of two hundred feet a year, while the 
margins may not advance more than fifteen or 
ten feet, or even less, in the same time. These 
observations of mine have the advantage over 
those of other observers, that, while they embrace 
the whole extent of the glacier, transversely as 
well as in its length, they cover a period of sev- 
eral successive years, instead of being limited to 
summer campaigns and a few winter observations. 
The consequence of this mode of progressing will 
be that the straight lines drawn transversely across 
the surface of the glacier 
above will be gradually 
changed to curved ones 
below. After a few years, 
such a line will appear on 
the surface of the glacier 
like a crescent, with the 
bow turned downward, 
within which, above, are 
other crescents, less and 
less sharply arched up to 
the last year’s line, which 
may be again straight 
across the snow-field. (See 
the subjoined figure, which 
represents a part of the 
glacier of the Lauter-Aar.) 
Thus the glacier records upon its surface its 
