124 PHYSICAL GEOGRAIHT OF THE SKA, AND ITS METEOKOLOGY. 



Tims we see how the top of tlie Andes becomes the reservoir 

 from which are supplied the rivers of Chili and Peru. The 

 other rainless or almost rainless regions are the western coast of 

 Mexico, the deserts of Africa, Asia, North America, and Australia. 

 Now study the geographical features of the country surrounding 

 those regions ; see how the mountain ranges run ; then turn to 

 Plate YIII. to see how the winds blow, and where the sources 

 are (§ 276) which supply them with vapours. This Plate shows 

 the prevailing direction of the wind only at sea ; but, knowing 

 it there, we may infer what it is on the land. Supposing it to 

 l^revail on the land as it generally does in corresponding latitudes 

 at sea, then the Plato will suggest readily enough how the winds 

 that blow over these deserts came to be robbed of their moisture, 

 or, rather, to have so much of it taken from them as to reduce 

 their dew-point below the Desert temperature ; for the air can 

 never deposit its moisture when its temperature is higher than its dew- 

 point. We have a rainless region about the Red Sea, because 

 the Eed Sea, for the most part, lies within the north-east trade- 

 wind region ; and these winds, when they reach that region, are 

 dry winds, for they have as yet, in their course, crossed no wide 

 sheets of water from which they could take up a supply of 

 vapour. Most of New Holland lies within the south-east trade- 

 wind region ; so does most of intertropical South America. But 

 intertropical South America is the land of showers. The largest 

 rivers and most copiously watered country in the world are to 

 be found there, whereas almost exactly the reverse is the case in 

 Australia. Whence this difference ? Examine the direction of 

 the winds with regard to the shore-line of these two regions, 

 and the explanation will at once be suggested. In Australia — ■ 

 east coast — the shore-line is stretched out in the direction of the 

 trades; in South America — east coast — it is perpendicular to 

 their direction. In Australia they fringe this shore only with 

 their vapour ; thus that thirsty land is so stinted with showers 

 that the trees cannot afford to spread their leaves out to the sun, 

 for it evaporates all the moisture from them ; their vegetable 

 instincts teach them to turn their edges to his rays. In inter- 

 tropical South America the trade- winds blow perpendicularly 

 upon the shore, penetrating the very heart of the country with 

 their moisture. Here the leaves, measuring many feet square — 

 as the plantain, &c. — turn their broad sides up to the sun, and 

 €0urt his rays. 



