EXTERNAL APPEARANCE OF GLACIERS. 280 
when boring a hole in the glacier in order to let 
down a self-regulating thermometer into its in- 
terior, seeing an immense fissure suddenly rent 
open, in consequence, no doubt, of the shocks 
given to the ice by the blows of the instruments. 
The effect was like that of an earthquake ; the 
mass seemed to rock beneath us, and it was dif- 
ficult to keep our feet. One of these glacial 
rivers was flowing past the spot at the time, and 
it was instantly lost in the newly formed chasm. 
However deep and wide the fissure might be, 
such a stream of water, constantly poured into 
it, and daily renewed throughout the summer, 
must eventually fill it and overflow, unless it 
finds its way through the whole mass of the gla- 
cier to the bottom on which it rests ; — it must 
have an outlet above or below. The fact that 
considerable rivulets (too broad to leap across, 
and too deep to wade through safely even with 
high boots) may entirely vanish in the glacier 
unquestionably shows one of two things, — that 
the whole mass must be soaked with water like 
a wet sponge, or the cavities must reach the bot- 
tom of the glacier. Probably the two conditions 
are generally combined. 
In direct connection with the narrower fissures 
are the so-called moulins , — the circular wells on 
the glacier already alluded to when speaking of 
the sand-hills. We will suppose that a trails 
