chap, viii.] THE CAUSES OF GLACIAL EPOCHS. 
141 
depends upon it. South 'Georgia in the latitude of Yorkshire 
is almost, and Sandwich Land in the latitude of the north of 
Scotland, is entirely covered with perpetual snow ; yet in their 
summer the sun is three million miles nearer the earth than it 
is in our summer, and the heat actually received from the sun 
must he sufficient to raise the temperature 20° F. higher than 
in the same latitudes 4n the northern hemisphere, were the con- 
ditions equal — instead of which their summer temperature is 
probably full 20° lower. The chief cause of this can only he 
that the heat of the sun does not reach the surface of the earth ; 
and that this is the Tact is testified by all Antarctic voyagers. 
Darwin notes the cloudy sky and constant moisture of the 
southern part of Chile, and in his remarks on the climate and 
productions of the Antarctic islands be says : “ In the Southern 
Ocean the winter is not so excessively cold, but the summer is 
far less hot (than in the north), for the clouded sky seldom allows 
the sun to warm the ocean, itself a bad absorbent of heat ; and 
hence the mean temperature of the year, which regulates the 
zone of perpetually congealed under soil, is low.” Sir James 
Ross, Lieutenant Wilkes, and other Antarctic voyagers speak 
of the snow-storms, the absence of sunshine, and the freezing 
temperature in the height of summer; and Dr. Croll shows 
that this is a constant phenomenon accompanying the presence 
of large masses of ice in every part of the world. 1 
In reply to the objections of a recent critic Dr. Croll has 
given a new proof of this important fact by comparing the 
known amount of snow-fall with the equally well-known melting 
power of direct sun-heat in different latitudes. He says : “ The 
annual precipitation on Greenland in the form of snow and rain, 
according to Dr. Rink, amounts to only twelve inches, and two 
inches of this he considers is never melted, but is carried away 
in the form of icebergs. The quantity of heat received at the 
equator from sunrise to sunset, if none were cut off by the 
atmosphere, would melt 3^ inches of ice, or 100 feet in a year. 
The quantity received between latitude 60° and 80°, which is 
1 For numerous details and illustrations see the paper — <l On Ocean 
Currents in relation to the Physical Theory of Secular Changes of Climate 
— in the Philosophical Magazine, 1870. 
