12 



TROPICAL NATURE, AND OTHER ESSAYS. 



south winds, except as local land and sea breezes, are 

 almost unknown. The Batavia observations show, that 

 for ten months in the year the average direction of the 

 wind varies only between 5° and 30° from due east or 

 west, and these are also the strongest winds. In the 

 two months — March and October — when the winds are 

 northerly, they are very light, and are probably in great 

 part local sea-breezes, which, from the position of 

 Batavia, must come from the north. As a rule, therefore, 

 every current of air at or near the equator has passed 

 obliquely over an immense extent of tropical surface 

 and is thus necessarily a warm wind. 



In the north temperate zone, on the other hand, the 

 winds are always cool, and often of very low temperature 

 even in the height of summer, due probably to their 

 coming from colder northern regions as easterly winds, or 

 from the upper parts of the atmosphere as westerly winds ; 

 and this constant supply of cool air, combined with quick 

 radiation through a dryer atmosphere, carries off the 

 solar heat so rapidly that an equilibrium is only reached 

 at a comparatively low temperature. In the equatorial 

 zone, on the contrary, the heat accumulates, on account 

 of the absence of any medium of sufficiently low 

 temperature to carry it off rapidly, and it thus soon 

 reaches a point high enough to produce those scorching 

 effects which are so puzzling when the altitude of the 

 sun or the indications of the thermometer are alone con- 

 sidered. Whenever, as is sometimes the case, exceptional 

 cold occurs near the equator, it can almost always be 

 traced to the influence of currents of air of unusually 

 low temperature. Thus in July near the Aru islands, 

 the writer experienced a strong south-east wind which 



