PROGRESSION OF GLACIERS. 
Fahrenheit in consequence of the infiltration of 
liquid water, to which the effects of pressure 
must be added, the importance of which in this 
connection no one could have anticipated prior 
to the experiments of Dr. Tyndall. Of course, 
if the interior temperature of the glacier never 
falls below 32°, the changes here alluded to could 
not take place. But if the vacuous spaces ob- 
served by Dr. Tyndall are really identical with 
the spaces I have described as extremely flattened 
air-bubbles , I think the arrangement of these 
spaces as above described proves that it freezes 
in the interior of the glacier to the depth at 
which these crosswise fragments have been ob- 
served ; that is, at a depth of two hundred feet. 
For, since the experiments of Dr. Tyndall show 
that the vacuous spaces are parallel to the sur- 
face of crystallization, and as no crystallization 
of water can take place unless the surrounding 
temperature fall below 32°, it follows that these 
vacuous spaces could not exist in such large con- 
tinuous fragments, presenting throughout the 
fragments the same trend, if there had been 
no frost within the mass, affecting the whole 
of such a fragment while it remained in the same 
position. 
The most striking evidence, in my opinion, 
that at times the whole mass of the glacier actu- 
ally freezes, is drawn from the fact, already allu- 
