Woeikof — CrolVs Hypotheses of Geological Climates. 175 



northern hemisphere, and the reverse during winter in aphelion 

 in the southern hemisphere. Why then are the trades of the 

 southern hemisphere blowing in the northern at present and 

 also why are the conditions more favorable to glaciation in the 

 southern than the northern hemisphere? There are certainly 

 good reasons for that. 



1. The extent and depth of the oceans of the southern hemi- 

 sphere. This gives a greater steadiness and force to the winds 

 of that hemisphere, and the difference is even more marked if 

 we compare the westerly winds of middle latitudes rather than 

 the trades, though also well seen in the latter. Now land acts 

 in two ways on the trade-winds ; it weakens them, first, by the 

 increase of friction. But this is not all. The trades, few ocean 

 regions excepted, are not strong winds, they are important on 

 account of their extent and steadiness. The gradient which 

 causes them is small. Now in such cases. land, even if it is not 

 a continent but only a cluster of small islands, has a great influ- 

 ence on trade-winds in causing local gradients which may have 

 even an opposite direction to the general gradients, thus causing 

 different and even opposing winds. The land- and sea-breezes 

 and the monsoons are cases in point. Even where the disturb- 

 ances of the normal ocean gradients are not large enough to 

 cause monsoons, we see generally the trades oftener interrupted 

 in summer, when they are weaker and when local thunder- 

 storms and rains are more frequent on land. For the two rea- 

 sons 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 to 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 the middle latitudes, the evaporation goes on 

 at a higher temperature from the seas of the northern, than 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 resulting pre- 

 cipitation will be snow and not rain even on low lands ; 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 favorable to glaciation. If they are 

 surrounded by land on which the winters have a temperature 

 considerably below 32°, they will be covered with ice, and thus 

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

 favorable to snowfall. The ice of the seas 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 favor- 



