Clouds are formed by the partial condensation of vapour, and are borne along by the wind, instead of falling as rain; the reason of their not falling is this, when the air below the clouds is saturated with moisture it will absorb no more and the rain falls, but when it is warm and dry, and passed in a constantly renewed current, then the vapour is absorbed before it reaches the earth, and is carried off by the wind. Clouds, therefore, although they often appear stationary, are constantly altering their form and size, portions being absorbed while others are being formed. Fogs are the same as clouds, the vapour arising from wet grass, rivers, &c., being condensed as it ascends, by a current of cold air passing over them. The fogs in London have a brown colour, from admixture of smoke, dust, &c., with which the air is contaminated (fig. 5). Clouds are of various forms and sizes, and indeed of almost every variety, but certain kinds which are frequently seen, have received different names. The "cirrhus" comprises all the feathery white clouds which float high up in the air in fine weather; the "cumulus" consists of the large mountain-like clouds which are generally seen in summer; the "stratus," those horizontal layers of clouds low down in the horizon, so often seen at sunset; and the "nimbus" is the rain-cloud, of a dark grey or leaden hue, with sharp well-marked edges (fig. 6). [Illustration: FIG. 7.] Clouds are amongst the most beautiful as well as useful things in nature, and it is one of the greatest proofs of the active benevolence of God, that all those things which serve man the most, are the most beautiful to contemplate. Without clouds there would be no rain, and without this no vegetation. In many parts of the tropical regions there is little or no rain, and in such parts desert places abound. Clouds are often of different states of electricity, and when they come near enough to each other, a transfer of the fluid takes place, accompanied with a flash of lightning (fig. 7) and a report, although this is not always heard at the same time that the lightning is seen, as sound does not travel nearly so fast as light; there is no danger from this kind of lightning. But it sometimes happens that a cloud in an opposite state of electricity to the surface of the earth is near enough to produce a flash of lightning between them, in this case it is extremely dangerous to be near; but when the thunder is not heard till some time after the flash is seen, there can be no danger, as it is then far away. Sound travels at about eleven hundred feet per second, therefore (in round numbers) it may be known that the lightning is one mile distant for every five seconds that elapse between the flash and the thunder. The notion which prevails that iron and steel attract lightning is entirely erroneous, they do not even conduct it so well as copper and many other metals. [Illustration: FIG. 8.]