404 NOTES OF A NATURALIST. 



between the equatorial and the colder zones. Taking the 

 surface of the earth generally, the trade-winds of the southern 

 are probably stronger than those of the northern hemisphere, 

 and, if it were true that the south temperate and frigid zones were 

 colder than those of the other hemisphere, it would be allowable 

 to argue that the greater difference of temperature as compared 

 with the equatorial zone was the cause of the greater strength 

 of the trade-winds. But we now certainly know that the 

 southern hemisphere between latitudes 45° and 55" is con- 

 siderably warmer than the corresponding zone of the northern 

 hemisphere, and we have good grounds for believing that the 

 mean temperature of the whole hemisphere south of latitude 45° 

 is higher, and certainly not lower, than that of the same portion 

 of the northern hemisphere. We are therefore not justified in 

 explaining the greater strength of the southern trade-winds by 

 a greater inequality of temperature between the equator and 

 the pole. 



In my opinion the cause of this predominance of the southern 

 trade-winds is to be sought in the fact that the southern is 

 mainly a water hemisphere, while the northern is in great part 

 a land hemisphere. In the south, the great currents of the 

 atmosphere flow with scarcely any interruption, except that 

 caused by Australia, where, in fact, the trade-winds are irregular, 

 and lose their force. In the northern hemisphere the various 

 winds originating in the unequal heating of the land surface 

 interfere with the normal force of the trade-winds, and weaken 

 their effect. 



In connection with this branch of the subject, I may remark 

 that the belief in the greater cold of the southern hemisphere 

 mainly rests on the fact that all the land hitherto seen in 

 high latitudes has been mountainous, and is covered by great 

 accumulations of snow and ice. But this does not in itself 

 justify the conclusion that the mean temperature is extremely 

 ow. It is true that the fogs which ordinarily rest on a snow- 

 covered surface much diminish the effect of solar radiation 

 during the summer in high latitudes, but this is compensated 

 by the great amount of heat liberated in the condensation of 

 vapour. The only part of the earth which is now believed to 

 be covered with an ice-sheet is Greenland, but the mean of the 

 observations in that country shows a temperature higher by at 



