PRESSURE OF TIDAL GLACIERS 215 



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- FIG. 106. IDEAL SEC- 

 duit, communicating the static pres- TION OF TIDAL GLACIER ' 



_ Showing relation of the 



sure of the sea water. If it were, the sea to the subgiadai water- 



i * , v j i_ film. The thickness of the 



ice would not be supported, because film is enormously ex a gg er- 

 that pressure is less than the down- ated - 

 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 



