CHAP. Yiii.J 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 be sufficient to raise the temperature 20° F. higher than 

 in the same latitudes in 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 be 

 that the heat of the sun does not reach the surface of the earth ; 

 and that this is the fact 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 he says: "In the Southern 

 Ocean the winter is not so excessively cold, but the summer is 

 far less hot (than in the north), /or the clouded shy seldom allows 

 the sun to ivarm 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.^ 



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 J 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 — On Ocean 

 Currents in relation to the Physical Theory of Secular Changes of Climate '' 

 — in the Philosophical Magazine, 1870. 



