6 ASPECTS OF TROPICAL NATURE 



appears i\8 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 northern sea, the atmospherical pre- 

 cipitations (dew, rain) caused by the cooling of the air are far 

 more abundant in the torrid zone than in the temperate lati- 

 tudes. 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, and 

 the annual fall of rain in the West Indies and on the coast of 

 Malabar rises to the enormous height of 274 and 283 inches. 

 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 

 fertiUty 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 examination, 

 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 



