4 THE TROPICAL WORLD. 



appears as a naked waste or decked with the most gorgeous 

 vegetation. 



As the evaporation of the tropical ocean is far more con- 

 siderable than that of the sea in higher latitudes, the atmo- 

 spherical precipitations (dew, rain) caused by the cooling of the 

 air are far more abundant in the torrid zone than in the tem- 

 perate regions of the earth. While the annual fall of rain 

 within the tropics amounts, on an average, to about eight 

 feet, it attains in Europe a height of only thirty inches ; 

 and under the clear equatorial sky the dew is often so abundant 

 as to equal in its effects a moderate shower of rain. 



But this enormous mass of moisture is most unequally dis- 

 tributed ; for while the greater part of the Sahara and the 

 Peruvian sand-coast are constantly arid, and South Africa 

 and North Australia suffer from long-continued droughts, we 

 find other tropical countries refreshed by almost daily showers. 

 The direction of the prevailing winds, the condensing powers of 

 high mountains and of forests, the relative position of a country, 

 the nature of its soil, are the chief causes which produce an 

 abundance or want of rain, and consequently determine, the 

 fertility or barrenness of the land. Of these causes, the first- 

 mentioned is by far the most general in its effects — so that a 

 knowledge of the tropical winds is above all things necessary 

 to give us an insight into the distribution of moisture over the 

 equatorial world. 



I have already mentioned the trade winds, or cool reactionary 

 currents called forth by the ascending equatorial air-stream ; 

 but it will now be necessary to submit them to a closer exami- 

 nation, and follow them in their circular course throughout the 

 tropical regions. In the Northern Atlantic, their influence, 

 varying with the season, extends to 22° N. lat. in winter, and 

 39° N. lat. in summer ; while in the southern hemisphere they 

 reach no farther than 18° S. lat. in winter, and 28° or 30° S. lat. 

 in summer. 



In the Pacific, their limits vary between 21° and 31° N. lat., 

 and between 23° and 33° S. lat.; so that, on the whole, they have 

 here a more southern position, owing, no doubt, to the vast 

 extent of open sea ; while in the Atlantic the influence of the 

 neighbouring continents forces them to the north, and even 

 causes the trade winds of the southern hemisphere to ascend 



