214 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 



