476 THE TROPICAL WORLD. 



there is a regular alternation of long periods of rain and drought, there is at different 

 seasons a wonderful interchange between apparent desolation and a profusion of vege- 

 table and animal life. The aspects of these regions will be more fully described 

 hereafter. 



The .great mountain ranges run mainly north and south. The great atmospheric 

 currents at the surface of the earth in tropical regions blow mainly from east to west. 

 The moisture in these winds is condensed when they strike the cold mountains, and 

 descends in the form of rain or snow. Hence, in general, the eastern sides of tropical 

 mountains are better watered than the western slopes. 



We have spoken of the trade winds as extending over the whole breadth of the 

 Tropical World. But to this there is a notable exception. Near the equator, but a 

 little to the north of it, the two currents from the Arctic and Antarctic regions meet 

 and neutralize each other, producing a belt of calms, which sailors call the "Dol- 

 drums," of about six degrees in breadth. Here it rains almost every day during the 

 year, for the ascending currents of heated air loaded with moisture become suddenly 

 cooled in the higher regions, and are forced to give up the water which they have 

 lifted from the ocean. Towards noon dense clouds form in the sky, and dissolve in 

 torrents of rain. Towards evening the vapors disperse, and the sun sets in a cloudless 

 horizon. The quantity of rain which here falls during the year is enormous. In the 

 United States the annual rainfall is from 25 to 70 inches; in Europe from 15 to 104; 

 in the Atlantic doldrums it reaches 225. So copious is the rainfall at times that fresh 

 water has been dipped up from the surface of the tropical seas. 



Proceeding north or south from the belt of calms, we come to a region characterized 

 by two rainy and two dry seasons. The rainy seasons take place while the sun is 

 passing the zenith, and more or less neutralizing the influence of the trade winds. In 

 Jamaica, for example, the first rainy season begins in April, the second in October; 

 the first dry season in June, the second in December. Towards the verge of the 

 tropics follow the zones characterized by a single rainy and a single dry season ; the 

 rains lasting from the vernal to the autumnal equinox. 



The two rainy seasons which characterize the middle zone between each tropic and 

 the equator have a tendency to merge into one rainy season of six months' duration on 

 advancing toward the tropics, and into a perpetual rainy season on approaching the 

 equator. As the sun goes north or south he opens the flood-gates of the heavens, and 

 closes them behind him as he passes to the other hemisphere ; while he keeps them 

 continually open where he is always vertical. But this general state of things, which 

 would be the normal condition of the tropical regions if their surface was an unbroken 

 sheet of water, and no disturbing forces existed, is liable to great modifications. 

 Thus in the monsoon region, extending from the eastern coast of Africa to the northern 

 part of Australia, and from the tropic of Capricorn to the Himalayas and China, it is 

 not the sun directly, but the winds that regulate the periodical rains. Thus in India 

 and the Malayan peninsula the western coasts are watered during the south-west mon- 

 soon, which prevails from April to October ; and the eastern coasts during the north- 

 east monsoon, from October to April. For example, the south-west wind condenses 

 its vapor on the western side of the Ghauts, the north-east on the eastern, so that 

 violent rains fall daily on the coast of Coromandel, while it is the reverse on that of 

 Malabar, and vice versa. In the southern hemisphere the rainy season corresponds with 



