i4 
Thickness of the Antarctic Ice , and its [January, 
mainly determined by the mean temperature of the upper 
surface of the sheet. All the ice down to the bottom of the 
sheet originally came from the surface. It once existed a 
the surface in the form of a coating of snow, which, be 
coming consolidated into ice, was afterwards covered over 
with fresh layers of snow, while these in turn passing into 
ice, were buried under succeeding snows, and so on. The 
ice that formed the surface a century ago now lies buried 
below the ice of a hundred years, and a hundred years hence 
its present position will be occupied by the surface ice of to- 
day. There is not only a constant motion of the ice from 
the pole outwards, but a constant downward motion as layer 
hy layer is successively formed on the surface. 
From what has been proved regarding the small quantity 
of heat which can be diredtly transmitted through the ice, 
it follows that the superficial layers will carry down with 
them pretty much the same temperature which they pos- 
sessed at the surface at the time when they were covered 
up by succeeding snows. Any heat which they can derive 
from the work of compression, as has been shown, is but 
trifling. Heat transmitted by conduction could not possibly 
raise the temperature of the underlying ice above that of 
the surface; neither could the heat from diredt radiation, 
nor that derived from melted ice. 
As the temperature of the ice, then, cannot be much 
above the mean temperature of the surface, which is far 
below the freezing-point, it follows that the underlying mass 
must also be below the freezing-point. The very low tem- 
perature of the superficial layers is due to the fadt that the 
mean temperature of the air above the surface is far below 
the freezing-point — a temperature which the icy surface 
cannot much exceed. The sun during summer may possibly 
heat the air sometimes above the freezing-point, but it can- 
not, of course, so raise the temperature of the ice without 
melting it. 
Again, as solid ice is a better radiator than gaseous air, 
the surface of the sheet during winter would probably have 
its temperature lowered by radiation to a greater extent 
than the air. The probability is that the mean annual 
temperature of the surface is as low as, if not lower than, 
that of the air over it. And although the mean tempera- 
ture of the regions around the South Pole has not been 
ascertained by diredt observation, yet it certainly cannot be 
higher, but is probably much lower* than, that of those 
around the North Pole, which we know is but a, few 
degrees above zero, F. 
* See Climate and Time, p. 63. 
