Although Africa is thus perpetually exposed to a scorching sun, yet, if it were well watered, it would be highly productive, and not unlike a luxuriant garden. But, although heat and water give this exuberant fertility to any soil, we also know that, without water (the blood of the vegetable world), the richest land remains a _caput mortuum_--_rudis indigestaque moles_--an inert, lifeless mass. Water being, therefore, an element of such vital importance in the production of vegetation, it becomes necessary to take a very short practical view of the tropical rains which deluge the centre of Africa. During the half-yearly visits which the sun pays in succession to the torrid regions on the north and south of the line, the air, heated by his presence, becomes rarefied, and flies upward: its place is immediately filled; and thus a constant rush of air, or, as we call it, a trade-wind, is produced, which, being also influenced by the diurnal motion of the sun, is constantly flowing towards the equator. The air, thus rushing towards the sun, is by heat made capable of absorbing a greater quantity of water than it could contain in a colder state; and therefore, as soon as this air and vapour united rise into high and consequently colder regions, a divorce between the two elements suddenly takes place; the air now loses its power of retaining the vapour, which, being immediately condensed, becomes water; and its companion, the dry air, thus deserted by it, falls to the earth in what we term tropical rains, which, accompanying the sun from one torrid zone to another, are, by a most wonderful provision of Nature, perpetually assuaging the thirst which this immense heating mass tends to create. The rains are always most violent where the sun is in the zenith; and, as a remarkable instance of the effect which they produce, it may be stated, that Bruce observed, when the sun was immediately over Gondar, the capital of Abyssinia, that the thermometer was invariably about twelve degrees lower than when he was in the southern tropic, thirty-six degrees from the zenith of Gondar: so happily does the approach of rain compensate for the heat of a burning sun! But, while the centre of Africa, or, to speak more correctly, a belt of about eleven hundred miles on each side of the line, is thus periodically deluged with water, yet, in the vast remainder of the continent, it may be said, with very few exceptions, that it never rains at all. The burning heat and the unequal distribution of water in Africa being understood, the following picture of the country is the natural consequence.