28 TROPICAL NATURE, AND OTHER ESSAYS. 



thousand feet, as in some of the volcanic mountains 

 of Java and on portions of the Eastern Andes. Beyond 

 the forests both to the north and south, we meet first 

 with woody and then open country, soon changing into 

 arid plains or even deserts which form an almost con- 

 tinuous band in the vicinity of the two tropics. On 

 the line of the tropic of Cancer we have, in America 

 the deserts and dry plains of New Mexico ; in Africa the 

 Sahara ; and in Asia, the Arabian deserts, those of Beloo- 

 chistan and Western India, and further east the dry 

 plains of North China and Mongolia. On the tropic of 

 Capricorn we have, in America the Grand Chaco desert 

 and the Pampas ; in Africa the Kalahari desert and the 

 dry plains north of the Limpopo ; while the deserts and 

 waterless plains of Central Australia complete the arid zone. 

 These great contrasts of verdure and barrenness occurring 

 in parallel bands all round the globe, must evidently 

 depend on the general laws which determine the distri- 

 bution of moisture over the earth, more or less modified 

 by local causes. Without going into meteorological 

 details, some of which have been given in the preceding 

 chapter, the main facts may be explained by the mode 

 in which the great aerial currents are distributed. The 

 trade winds passing over the ocean from north-east to 

 south-west with an oblique tendency towards the equator, 

 become saturated with vapour, and are ready to give 

 out moisture whenever they are forced upwards or in any 

 other way have their temperature lowered. The entire 

 equatorial zone becomes thus charged with vapour-laden 

 air which is the primary necessity of a luxuriant vege- 

 tation. The surplus air (produced by the meeting of the 

 two trade \vinds) which is ever rising in the equatorial 



