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.