266 
INTERNAL STRUCTURE AND 
is accelerated in spring and early summer. The 
average annual advance of the glacier being, at 
a given point, at the rate of about two hundred 
feet, its average summer advance, at the same 
point, will be at a rate of two hundred and fifty 
feet, while its average rate of movement in win- 
ter will be about one hundred and fifty feet. This 
can be accounted for only by the increased press- 
ure due to the large accession of water trickling 
in spring and early summer into the interior 
through the network of capillary fissures per- 
vading the whole mass. The unusually large in- 
filtration of water at that season is owing to the 
melting of the winter snow. Careful experiments 
made on the glacier of the Aar, respecting the 
water thus accumulating on the surface, pene- 
trating its mass, and finally discharged in part 
at its lower extremity, fully confirm this view. 
Here, then, is a powerful cause of pressure and 
consequent motion, quite distinct from the per- 
manent weight of the mass itself, since it oper- 
ates only at certain seasons of the year. In mid- 
winter, when the infiltration is reduced to a 
minimum, the motion is least. The water thus 
introduced into the glacier acts, as we have seen 
above, in various ways : by its weight, by loosen- 
ing the particles of snow and ice through which 
it trickles, and by freezing and consequent ex- 
pansion, at least within the limits and during the 
