Physical Theory of Secular Changes of Climate. 85 
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 
use which would tend to keep the snow at a low tempera- 
ture would be that, along with a cold northerly wind, there is 
usually a great diminution of aqueous vapor, thus allowing the 
surface of the snow to radiate its heat more freely into stellar 
space. For were it not for the aqueous vapor in the atmos- 
phere, 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 inconsiderable 
snowfall, is not at the present day covered with permanent 
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 dimin- 
ished 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 
_ *, * See American Journal of Science for October, 1883; Philosophical Magazine 
for October, 1883. 
+ Phil. Mag. for N ovember, 1883. 
