238 Dr. A. Woeikof 's Examination of 



trades oftener interrupted in summer, when they are weaker, 

 and when local thunderstorms aud rains are more frequent on 

 land. For the two reasons given, the trades of the southern 

 hemisphere must be more extensive and stronger than those 

 of the northern. 



2. The relatively small extent of sea in middle latitudes of 

 the northern hemisphere in comparison with the southern, must 

 tend to warm the seas of the former, even if the quantity of 

 warm water from the tropical seas reaching them be equal. 

 Thus, generally in middle latitudes, the evaporation goes on at 

 a higher temperature from the seas of the northern than of the 

 southern hemisphere. Now, this has a very great influence 

 on the resulting precipitation; when the evaporation goes on 

 at or near 32°, there is much more probability that the result- 

 ing precipitation will be snow and not rain even on lowlands ; 

 the higher the temperature at which the evaporation takes 

 place, the greater must be the height at which snow can fall, 

 on account of cooling by expansion. 



3. Not all cold seas are favourable to glaciation. If they 

 are surrounded by land on which the winters have a tempe- 

 rature much below 32°, they will be covered with ice, and 

 thus evaporation will be checked just at the time when it is the 

 most favourable to snowfall; the ice of the sea will be covered 

 with snow; the temperature of the air over it may be very low, 

 but the snowfall will not be great; and thus the conditions 

 not favourable to glaciation. Such is the condition of many 

 seas of the northern hemisphere, as the Arctic Ocean north of 

 Siberia, the Kara Sea, the bays and inlets north of the North- 

 American continent, the sea of Okhotsk, &c, which are 

 covered with ice for many months. These conditions are 

 favourable to a long and cold winter, but not to a large snow- 

 fall and the resulting glaciation. The observations made at 

 many points of the coasts of Siberia and the North- American 

 archipelago have shown, that the snowfall is exceedingly 

 light. The seas of the higher southern latitudes are deep and 

 not surrounded by land, and thus by far not so ice-bound, 

 both on account of the absence of very low temperatures 

 favourable to the formation of ice, and of the rupture of the 

 ice, when formed, by winds and currents. Such seas as 

 these are favourable to snowfall and glaciation on land, as 

 even in midwinter there is a great extent of water which 

 evaporates freely. The only parts of the northern hemi- 

 sphere where glaciation is considerable outside of high moun- 

 tains, is the region from Greenland to Francis- Joseph Land; 

 but here we have rather cold seas, which are yet not entirely 

 ice-bound even in winter. These seas are more favourable 



