sequenlly placing the glacial period of either hemisphere in 

 that portion of the cycle where the aphelion occurs in the 

 winter. 



There is one conspicuous fact of the present physical geography 

 of the earth which appears to support Mr. Croll's opinion. 

 The earth's aphelion occurs at present in the winter of the 

 southern hemisphere, and that hemisphere is the most glaciated 

 of the two ; — the entire Antarctic Continent is covered with an 

 ice-cap down to the water's edge ; while in Greenland, on the 

 contrary, the covering of continental ice in most places does 

 not come down to the sea, and in Siberia and North America 

 there is no continental ice at all. This however is amply 

 accounted for by the great extent of ocean in the southern 

 hemisphere. Water has the greatest capacity for heat of all 

 known substances, and consequently takes a long time to heat 

 and a long time to cool. For this reason masses of water tend 

 to equalize the seasons, mitigating both summer heat and 

 winter cold ; and the depression of summer temperature in the 

 southern hemisphere, due to this cause, lowers the snow-line 

 and promotes glaciation. 



But if land and water were equally distributed over both 

 hemispheres, I cannot doubt that the fact of the perihelion 

 occurring in the summer of the southern hemisphere would 

 raise the summer temperature of that hemisphere, and cause it 

 to be less glaciated than the northern. This is the state of 

 things at present in the planet Mars. The proportion of land 

 to ocean is greater in Mars than in the earth, and they are 

 nearly equally distributed over the two hemispheres. In Mars, 

 as in the earth, the perihelion at present coincides with mid- 

 winter of the northern and midsummer of the southern hemi- 

 sphere ; and the excentricity of his orbit is somewhat greater 

 than that of the earth at its maximum. We ought conse- 

 quently to expect the extent of permanent snow to be much 

 less round the south than the north pole of Mars, and this is 

 what has been observed. Round each pole there is a permanent 

 snow-cap. The northern one was observed in 1837, and its 

 least diameter, which was shortly after the midsummer of that 



