g ASPECTS OF TROPICAL NATURE 



their eastern gorges with a perpetual verdure, their western slopes 

 are almost constantly arid. Such is the influence of this colossal 

 barrier in interrupting the course of the air-current, that the 

 trade wind only begins to be felt again on the Pacific at a dis- 

 tance of one hundred or even one hundred and fifty miles from 



the shore. 



In South Africa, also, we find the eastern mountainous coast- 

 lands covered with giant timber — in striking contrast with the 

 parched savannas or dreary wastes of the interior ; and in the 

 South Sea the difference of verdure between the east and west 

 coasts of the Galapagos, the Sandwich Islands, the P'eejees, and 

 many other groups, never fails to arrest the attention of the 

 mariner. 



The trade winds of the northern and southern hemispheres 

 do not, however, blow in one continuous stream over the whole 

 breadth of the tropical ocean, but are separated from each other 

 by a zone or belt of calms, occasioned by their mutually paralys- 

 ing each other's influence on meeting from the north and the 

 south-east, and by the attraction of the sun, which, when in 

 the zenith, changes the easterly air-currents into an ascending 

 stream. From this dependence on the position of the sun, it 

 may easily be inferred that the zone of calms fluctuates, like 

 the trade winds themselves, to the north or south, according to 

 the seasons ; and that it is far from invariably occupying the 

 same degrees of latitude, or the same width, at all times of the 

 year. In the Atlantic, from the causes previously mentioned, 

 it constantly remains to the north of the line, where its breadth 

 averages five or six degrees ; in the Pacific it more generally 

 extends, during the antarctic summer, on both sides of the 

 equator. 



Besides the intensity of its heat, the zone of calms is charac- 

 terised by heavy showers, which regularly fall in the afternoon, 

 and are caused by the cooling of the saturated air-columns in 

 the higher regions of the air. 



Daily, towards noon, dense clouds form in the sky, and dissolve 

 in torrents of rain under fearful electrical explosions, now sooner, 

 now later, of shorter or longer continuance, with increasing or 

 abating violence, as the sun is more or less in his zenith. 

 Towards evening the vapours disperse, and the sun sets in a 

 clear, unclouded horizon. Thus towns or countries situated 



