PRESSURE OF TIDAL GLACIERS 
2I 5 
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MS 
melting, and as the added water must escape by flow, 
there is also a dynamic factor, the viscous resistance to 
flow. These forces conjointly determine the thickness 
of the film. The film is thicker as the ice column is less, 
as the sea-water column is greater, and as the melting is 
more rapid. It is always very thin. 
We may now advantageously return to the question of 
the mode of support of the tidal glacier. Referring to the 
diagram (fig. 106), it is evident that the ice receives no 
support from the pressure of the sea 
water against its frontal cliff. It rests 
wholly on the film of water beneath 
it, and its pressure is communicated 
by the film, without loss, to the rock 
beneath. The film is not a mere con¬ 
duit, communicating the static pres¬ 
sure of the sea water. If it were, the 
ice would not be supported, because 
that pressure is less than the down¬ 
ward pressure of the ice. In virtue of the molecular forces 
brought into play along the contact planes, the film has 
some of the properties of a solid. It is, in some sense, an 
elastic spring or cushion interposed between the ice and 
the rock. It performs its function of transmitting the 
pressure of the glacier to the rock bed quite indepen¬ 
dently of the presence of the sea. The pressure of the 
sea water modifies the infinitesimal thickness of the film, 
but does not prevent the rock bed from supporting (through 
the mediation of the film) the whole weight of the glacier. 
The last statement is subject to a single qualification. 
Wherever the feeble flow of water from basal melting 
maintains passages of more than capillary size, the molec¬ 
ular forces locally cease to dominate, and the hydrostatic 
pressure of the sea water contributes support to the 
glacier. To this extent, which must always be relatively 
FIG. Io6. IDEAL SEC¬ 
TION OF TIDAL GLACIER. 
Showing relation of the 
sea to the subglacial water- 
film. The thickness of the 
film is enormously exagger¬ 
ated. 
