On the average, the rainfall over the oceans is much less than over the land, the whole region of the trade-winds having usually a cloudless sky and very little rain; but in the intervening belt of calms, near to the equator, a cloudy sky and heavy rains are frequent. This arises from the fact that the warm, moist air over the ocean is raised upwards, by the cold and heavy air from north and south, into a cooler region where it cannot hold so much aqueous vapour, which is there condensed and falls as rain. Generally, wherever the winds blow over extensive areas of water on to the land, especially if there are mountains or elevated plateaus which cause the moisture-laden air to rise to heights where the temperature is lower, clouds are formed and more or less rain falls. But if the land is of an arid nature and much heated by the sun, the air becomes capable of holding still more aqueous vapour, and even dense rain-clouds disperse without producing any rainfall. From these simple causes, with the large area of sea as compared with the land upon our earth, by far the larger portion of the surface is well supplied with rain, which, falling most abundantly in the elevated and therefore cooler regions, percolates the soil, and gives rise to those innumerable springs and rivulets which moisten and beautify the earth, and which, uniting together, form streams and rivers, which return to the seas and oceans whence they were originally derived. CLOUDS AND RAIN DEPEND UPON ATMOSPHERIC DUST