Thus we see how the top of the 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 XIII. 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 prevail 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 Red Sea, for the most part, lies within the north-east tradewind 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 court his rays.
