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 
the density of mercury is about five 
times that of glass, some force was 
needed to immerse the block of 
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 
—— 
— —=ji 
Pee— 
~— ■ ■ — ~ ~ 
-hH 
ijh 
iHr 
FIG. IO 5 . CONTACT PHENOM¬ 
ENA OF GLASS AND MER¬ 
CURY. 
A block of glass rests on the bot¬ 
tom ; a similar block floats 
at the surface. 
