The temperature of a country proportionally decreases with its elevation; and thus the high situation of many tropical lands moderates the effects of equatorial heat, and endows them with a climate similar to that of the temperate, or even of the cold regions of the globe. The Andes and the Himalaya, the most stupendous mountain-chains of the world, raise their snow-clad summits either within the tropics or immediately beyond their verge, and must be considered as colossal refrigerators, ordained by Providence to counteract the effects of the vertical sunbeams over a vast extent of land. In Western Tropical America, in Asia, and in Africa, we find immense countries rising like terraces thousands of feet above the level of the ocean, and reminding the European traveller of his distant northern home by their productions and their cooler temperature. Thus, by means of a few simple physical and geological causes acting and reacting upon each other on a magnificent scale, Nature has bestowed a wonderful variety of climate upon the tropical regions, producing a no less wonderful diversity of plants and animals.
But warmth alone is not sufficient to call forth a luxuriant vegetation: it can only exert its powers when combined with a sufficient degree of moisture; and it chiefly depends upon the presence or absence of water whether a tropical country appears as a naked waste or decked with the most gorgeous vegetation.
As the evaporation of the tropical ocean is far more considerable than that of the sea in higher latitudes, the atmospherical precipitations (dew, rain) caused by the cooling of the air are far more abundant in the torrid zone than in the temperate regions of the earth. While the annual fall of rain within the tropics amounts, on an average, to about eight feet, it attains in Europe a height of only thirty inches; and under the clear equatorial sky the dew is often so abundant as to equal in its effects a moderate shower of rain.
But this enormous mass of moisture is most unequally distributed; for while the greater part of the Sahara and the Peruvian sand-coast are constantly arid, and South Africa and North Australia suffer from long-continued droughts, we find other tropical countries refreshed by almost daily showers. The direction of the prevailing winds, the condensing powers of high mountains and of forests, the relative position of a country, the nature of its soil, are the chief causes which produce an abundance or want of rain, and consequently determine the fertility or barrenness of the land. Of these causes, the first-mentioned is by far the most general in its effects—so that a knowledge of the tropical winds is above all things necessary to give us an insight into the distribution of moisture over the equatorial world.
I have already mentioned the trade winds, or cool reactionary currents called forth by the ascending equatorial air-stream; but it will now be necessary to submit them to a closer examination, and follow them in their circular course throughout the tropical regions. In the Northern Atlantic, their influence, varying with the season, extends to 22° N. lat. in winter, and 39° N. lat. in summer; while in the southern hemisphere they reach no farther than 18° S. lat. in winter, and 28° or 30° S. lat. in summer.
In the Pacific, their limits vary between 21° and 31° N. lat., and between 23° and 33° S. lat.; so that, on the whole, they have here a more southern position, owing, no doubt, to the vast extent of open sea; while in the Atlantic the influence of the neighbouring continents forces them to the north, and even causes the trade winds of the southern hemisphere to ascend beyond the equatorial line. Their character is that of a continual soft breeze—strongest in the morning, remitting at noon, and again increasing in the evening. In the neighbourhood of the coasts, except over very small islands, they become weaker, and generally cease to be felt at a distance of about fifteen or twenty miles from the sea, though, of course, at greater heights they continue their course uninterruptedly over the land.
For obvious reasons the trade winds have been much more accurately investigated upon the ocean than on land, particularly in the Northern Atlantic, which is better known in its physical features than any other sea, as being a highway for numberless vessels to which the study of the winds is a matter of the greatest importance; yet, in spite of so many disturbing influences, their course, even over the continents, has been ascertained by travellers. North-easterly winds almost constantly sweep over the Sahara; and in South Africa, Dr. Livingstone informs us that north-easterly and south-easterly winds blow over the whole continent between 12° and 6° S. lat., even as far as Angola, where they unite with the sea winds.
In Brazil, the presence of the trade winds has been determined with still greater accuracy. Thus easterly breezes almost perpetually sweep over the boundless plains up to the slopes of the Andes, and even in Paraguay (25° S. lat.) a mild east wind constantly arises in summer after the setting of the sun.
As the trade winds originate in the coldest, and thence pass onwards to the warmer regions, they are, of course, constantly absorbing moisture as they advance over the seas. Saturated with vapours, they reach the islands and continents, where, meeting with various refrigerating influences (mountain-chains, forests, terrestrial radiation), their condensing vapours give rise to an abundance both of rain and dew. It is owing to their influence that in general, within the tropics, the eastern coasts, or the eastern slopes of the mountains, are better watered than the interior of the continents or lands with a western exposure.
An example on the grandest scale is afforded to us by South America, where the Andes of Peru and Bolivia so effectually drain the prevailing east winds of their moisture, that while numberless rivulets, the feeders of the gigantic Marañon, clothe their eastern gorges with a perpetual verdure, their western slopes are almost constantly arid. Such is the influence of this colossal barrier in interrupting the course of the air-current, that the trade wind only begins to be felt again on the Pacific at a distance of one hundred or even one hundred and fifty miles from the shore.