it EQUATORIAL VEGETATION 239 
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 Beloochistan and Western 
India, and farther 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 distribution 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, 
and from south-east to north-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 
vegetation. The surplus air (produced by the meeting of the 
two trade winds) which is ever rising in the equatorial belt 
and giving up its store of vapour, flows off north and south as 
dry, cool air, and descends to the earth in the vicinity of the 
tropics. Here it sucks up whatever moisture it meets with 
and thus tends to keep this zone in an arid condition. The 
trades themselves are believed to be supplied by descending 
currents from the temperate zones, and these are at first 
equally dry and only become vapour-laden when they have 
passed over some extent of moist surface. At the solstices 
the sun passes vertically over the vicinity of the tropics for 
several weeks, and this further aggravates the aridity ; and 
wherever the soil is sandy and there are no lofty mountain 
chains to supply ample irrigation, the result is a more or less 
perfect desert. Analogous causes, which a study of aerial 
currents will render intelligible, have produced other great 
