2I 4 
ALASKA GLACIERS 
for if these fall below the freezing temperature, a film of 
water can not exist between them. This is a subject 
which has received considerable attention, and although 
direct observation is impossible, there seems a good 
foundation for inference. By the aid of crevasses and 
borings it has been found that the upper ice (outside the 
neve region) has at all times a temperature of almost ex¬ 
actly 32°, seasonal and diurnal variation being confined 
to a very thin surface layer. The upper ice therefore has 
no cooling effect on the bottom ice. On the other hand, 
the bottom ice receives heat in three ways: Heat comes to 
it by conduction from the interior of the earth; heat is 
developed by the friction of ice and waste on the bed¬ 
rock; and the internal work of the flow of the ice devel¬ 
ops heat, of which a part is conducted downward. The 
bottom ice therefore maintains a temperature of 32° 
(more precisely, the freezing temperature corresponding 
to the pressure), and the adjacent rock is slightly warmer. 
There is a continual, though very slow, melting of the 
basal ice, and a film of water is maintained between it 
and the rock. It is probable that the streams of water 
which flow from glaciers all through the winter are sup¬ 
plied chiefly by basal melting; and we may further sup¬ 
pose that the tunnels through which those streams flow 
are connected with the thin water film by a graduated 
and ramifying system of minor passages. The ways 
which serve for the escape of the product of basal melting 
serve also, in the case of tidal glaciers, for the communi¬ 
cation of the hydrostatic pressure of the sea water. 
Statically considered, the film of water under the gla¬ 
cier is subject to a group of forces in equilibrium. The 
weight of the glacier presses on it and tends to expel it. 
This is resisted by the molecular forces associated with 
the contact faces of the film, and by the hydrostatic pres¬ 
sure of the sea water outside. As the film is added to by 
