LXXIV PRESIDENTIAL ADDRESS. 



great fact remains, of which use will be made presently, that the 

 more rapid the circulation of the atmosphere becomes from the 

 Pole to the Equator, the more is the atmospheric pressure in the 

 neighbourhood of the Pole lowered. Consequently, if any factors 

 which make for the acceleration of the atmospheric circulation are 

 at any time increased, the barometric pressure at the Poles will 

 be lower than ever, and the atmosphere there may become thinner 

 than ever. Such factors will be shown in the sequel to have 

 probably existed in the South Polar Region in Permo-Carboniferous 

 time. 



We may now pass on to another very important feature which 

 is indicated by the nature of the curves, and that is that the 

 North Polar summer is considerably warmer than the South Polar, 

 and the North Polar winter at sea-level considerably colder than 

 the South Polar winter at sea-level. 



The following reasons suggest them.selves to me as an explana- 

 tion of this important fact : — First, the superior cold of the North 

 Polar winter as compared with the South Polar winter, at sea- 

 level, is probably due to several factors, but the chief factor is 

 this — In the Northern Hemisphere the proportion of land to sea 

 is much greater than in the Southern Hemisphere. On account 

 of the much less specific heat of land than water, it follows that, 

 as winter approaches, the great continents around or outside of 

 the Arctic Circle radiate heat very rapidly and fall below freezing 

 point much earlier than does the sea in similar latitudes in the 

 corresponding winter months in the Southern Hemisphere ; if, 

 indeed, the sea in such latitudes as lie north of the Antarctic 

 Circle ever freezes over at all. 



Next, we find that in the coldest month of the Northern Hemi- 

 sphere — January — not only is the whole of the Arctic Ocean frozen, 

 but vast areas of North America, Northern Europe, and Siberia, 

 including Mongolia and China as far south as Corea, are enveloped 

 in snow. This area of earth surface, including sea ice, below 

 freezing point in winter in the Northern Hemisphere may be 

 estimated at approximately from 15 to 20 million square miles; 

 whereas in the Antarctic, inclusive of the narrow rim of sea ice 

 which forms during the winter, there can never, under existing 

 conditions, be much more than about 5,000,000 square miles under 

 ice. The central parts of the Antarctic Continent can nowhere be 

 more than about 1,000 miles distant from the nearest open water, 

 and the Anarctic Continent at this distance is entirely surrounded 

 by water, the temperature of which is about 30° P. The steep 

 isobaric grade, as already explained, leads to a rapid interchange, 

 especially in winter time, of relatively warm air from over the 

 ocean with intensely cold air lying over the Pole. 



