212 ALASKA GLACIERS 



tacitly assumed that the contact of the ice with the bed 

 excludes the water. In order to determine which analy- 

 sis is applicable to the case of the tidal glacier, it is neces- 

 sary to consider whether the nature of the contact between 

 the glacier and its bed is or is not such as to exclude the 

 sea water and its pressure. 



It was suggested by my colleague G. F. Becker that a 

 laboratory test might be applied to one of the principles 

 appealed to in the second of the two analyses the prin- 

 ciple that a solid rectangular block immersed in a liquid 

 is not buoyed up by the liquid provided its base is in com- 

 plete contact with the bed on which the liquid rests ; and 

 at his request two pertinent experiments were made by 

 A. L. Day in the physical laboratory of the U. S. Geolog- 

 ical Survey. In the first experiment a small slab of plate 

 glass was cemented to the bottom of a glass vessel (fig. 

 105), for the purpose of giving an accurately plane surface, 



and the vessel was then partly filled 

 with mercury. A second piece of 

 plate glass was immersed in the 

 mercury and pushed down until 

 one of its faces came into contact 

 with the face of the fixed slab. As 



FIG. 105. CONTACT 

 ENA OF GLASS AND MER- the density of mercury is about five 



times that of glass, some force was 



A block of glass rests on the bot- 



tom ; a similar block floats needed to immerse the block of 



at the surface. t , , 



glass, but as soon as contact had 



been secured with the slab below, the block remained at 

 the bottom. Not only did it show no tendency to rise, 

 but force was necessary to detach it. As both glass sur- 

 faces had been carefully cleaned, there could be no cemen- 

 tation; the phenomenon was one of hydrostatic pressure, 

 conditioned by the contact relations of glass with mer- 

 cury. 



In the second experiment water was substituted for 



