92 THE ANTARCTIC MANUAL. 
of comparative purity and high freezing-point, while in winter it is 
surrounded by a mere film of brine of comparatively low freezing- 
point The difference between land ice and sea ice is one of degree and 
not of kind. Sea ice is mixed with much brine and flows easily : land 
ice contains little brine and flows with difficulty. 
A ship floats in the smallest basin as perfectly as in the largest 
ocean, We can imagine a dock being built round a ship, and so 
exactly moulded to its shape, that between the inner surface of the 
dock and the outer surface of the ship, the clearance shall be so 
small that a pitcher of water poured into it will float the ship. The 
floating of a grain in the inside of a glacier is of this kind, and as 
it is enclosed on all sides, it will press against the ice above it in 
preference to that beneath it. 
This feature of glacier ice permits us to understand how glaciers 
can move, and begin to move, even when their temperature is very 
low. Before von Drygalski’s work on Greenland, we had no trust- 
worthy information regarding the temperature throughout the year 
of the inner mass of the ice of any glacier. He carried out, at 
recular intervals of time, a series of observations of the temperature 
at different depths below the surface of one of the Greenland glaciers, 
and parallel observations within the ice covering a neighbouring 
lake. These observations showed that the temperature of the glacier 
increases rapidly from the surface downward, and they render it 
probable that the greater part of the thickness of a Greenland glacier 
is, even at the coldest time of the year, at or near the ordinary 
temperature of melting ice.* The heat required to support this 
temperature can only be supplied by the friction of the grains of 
the ice, called into being by the motion of the glacier. The inland 
ice which forms the great reservoir for the supply of the glacier, was 
found to have little or no appreciable motion. Series of temperatures 
were not taken in its thickness, but, in the absence of motion, we 
may believe that the very low temperature at its surface penetrates 
far into its interior, if not to the very bottom. If the motion were 
dependent only on the lowering effect of pressure on the melting- 
point of pure ice, it would be difficult for such a mass of ice to start 
when it arrives at an outlet. The impurity of all natural water, and 
the effect which it has in lowering the melting-point of ice at 
ordinary pressures, remove this difficulty. However compact and 
solid the blue ice may look, there will always be some brine between 
its grains which will permit some yielding of its mass, which in its 
* Compare Hugi, ‘ Ueber das Wesen der Gletscher und Winterreise in das Eismeer,’ 
p. 51. 
