140 ISLAND LIFE parti 



The reason why no accumulation of snow or ice ever 

 takes i^lace on Arctic lowlands is explained by the observa- 

 tions'of Lieut. Payer of the Austrian Polar Expedition, who 

 found that during the short Arctic summer of the highest 

 latitudes the ice-fields diminished four feet in thickness 

 under the influence of the sun and wind. To replace this 

 would require a precipitation of snow equivalent to about 

 45 inches of rain, an amount which rarely occurs in low- 

 lands out of the tropics. 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, 

 probably the south-western anti-trades overcoming the 

 northern inflow ; and under its influence the snow all dis- 

 appears in a few days and the vegetable kingdom bursts 

 into full luxuriance. This is very important as showing 

 the imjDotence of mere sun-heat to get rid of a thick mass 

 of snow so long as the air remains cold, while currents of 

 warm air are in the highest degree effective. If, however, 

 they are not of sufficiently high temperature or do not 

 last long enough to melt the snow, they are likely to 

 increase it, from the quantity of moisture they bring with 

 them which will be condensed into snow by coming into 

 contact with the frozen surface. We may therefore expect 

 the transition from perpetual snow to a luxuriant arctic 

 vegetation to be very abrupt, depending as it must on a 

 few degrees more or less in the summer temperature of 

 the air; and this is quite in accordance with the fact of 

 corn ripening by the sides of alpine glaciers. 



Efficiency of Astronomical Causes in Producing Glacia- 

 tion. — Having now collected a sufficient body of facts, let 

 us endeavour to ascertain what would be the state to 

 which the northern hemisphere would be reduced by a 



no signs of having been largely glaciated. "West of the Rocky Mountains, 

 however, in the Sierra Nevada and the coast ranges further north, signs 

 of extensive old glaciers again appear ; all which phenomena are strikingly 

 in accordance with the theory here advocated, of the absolute dependence 

 of glaciation on abundant rainfall and elevated snow-condensers and 

 accumulators. 



