PROGRESSION OF GLACIERS. 273 
Fahrenheit, that is, four degrees below the freez- 
ing-point, at a depth of six feet below a thick 
covering of snow, though not absolutely conclu- 
sive as to the temperature at a greater depth, is 
certainly very significant. 
Under these circumstances, it is not out of 
place to consider through what channels the low 
temperature of the air surrounding the glacier 
may penetrate into the interior. The heavy cold 
air may of course sink from the surface into 
every large open space, such as the crevasses, 
large fissures, and moulins or mill-like holes to 
be described in a future article ; it may also pen- 
etrate with the currents which ingulf themselves 
under the glacier, or it may enter through its 
terminal vault, or through the lateral openings 
between the walls of the valley and the ice. In- 
deed, if all the spaces in the mass of the glacier, 
not occupied by continuous ice, could be graphi- 
cally represented, I believe it would be seen that 
cold air surrounds the glacier-ice itself in every 
direction, so that probably no masses of a greater 
thickness than that already known to be perme- 
able to cold at the surface would escape this con- 
tact with the external temperature. If this be 
the case, it is evident that water may freeze in 
any part of the glacier. 
To substantiate this position, which, if sus- 
tained, would prove that the dilatation of the 
12 * R 
