of Secular Changes of Climate. 85 



certain conditions, the snow-line may in some places have 

 been brought to the sea-level. In arctic, or even in subarctic 

 regions, an excessively heavy snowfall, followed by piercingly 

 cold winds from the north, during the whole of the summer 

 months, would keep the snow at a low temperature, and cer- 

 tainly prevent it from disappearing. Keep the surface of the 

 snow at or below the freezing-point, and melting will not take 

 place, no matter how intense the sun's rays may be. A strong- 

 wind below the freezing-point will cool the surface of the snow 

 more rapidly than the sun can manage to heat it. Another 

 cause which would tend to keep the snow at a low temperature 

 would be that, along with a cold northerly wind, there is 

 usually a great diminution of aqueous vapour, thus allowing 

 the surface of the snow to radiate its heat more freely into 

 stellar space. For were it not for the aqueous vapour in the 

 atmosphere, the snow-line, even at the equator, would descend 

 to the sea-level*. 



Perhaps it is owing to the warm southerly winds of the 

 two midsummer months that Siberia, even with its incon- 

 siderable snowfall, is not at the present day covered with per- 

 manent snow and ice. Mr. Wallace mentions that " in Siberia, 

 within and near the Arctic circle, about six feet of snow covers 

 the country all the winter and spring, and is not sensibly 

 diminished by the powerful sun so long as northerly winds 

 keep the air below the freezing-point, and occasional snow- 

 storms occur. But early in June the wind usually changes 

 to southerly, and under its influence the snow all disappears 

 in a few days." But what would be the consequence were 

 these northerly winds to continue during the whole of June 

 and July ? It would probably be that the snow of autumn 

 would begin to fall before that of spring had disappeared. 

 Were this to result, the country would soon become covered 

 with permanent ice. Matters would be still worse if these 

 southerly winds, instead of ceasing, were simply to change 

 from June and July to December and January, for then, in 

 place of producing a melting effect, they would greatly add 

 to the snowfall. 



Such a condition of things may never have obtained on the 

 plains of Siberiaf ; but I have shown in my paper on the Ice 

 of Greenland and the Antarctic regions^ that there are cer- 

 tainly good grounds for concluding that during the glacial 



* See American Journal of Science for October 1883 ; Philosophical 

 Magazine for October 1883. 



t Sir Joseph Dalton Hooker suggests to me that the Ice-clifls of Siberia 

 may, however, be relics of the Glacial Epoch. 



X Phil. Mag. for November 1883. 



