§ 13. The extreme whiteness of the volume of vapor in this case (not, I fear, very intelligible in the woodcut[3]) may be partly owing to recent rain, which, by its evaporation, gives a peculiar density and brightness to some forms of clearing cloud. In order to understand this, we must consider another set of facts. When weather is thoroughly wet among hills, we ought no more to accuse the mountains of forming the clouds, than we do the plains in similar circumstances. The unbroken mist buries the mountains to their bases; but that is not their fault. It may be just as wet and just as cloudy elsewhere. (This is not true of Scottish mountain, by the way.) But when the wet weather is breaking, and the clouds pass, perhaps, in great measure, away from the plains leaving large spaces of blue sky, the mountains begin to shape clouds for themselves. The fallen moisture evaporates from the plain invisibly; but not so from the hill-side. There, what quantity of rain has not gone down in the torrents, ascends again to heaven instantly in white clouds. The storm passes as if it had tormented the crags, and the strong mountains smoke like tired horses. § 14. Here is another question for us of some interest. Why does the much greater quantity of moisture lying on the horizontal fields send up no visible vapor, and the less quantity left on the rocks glorify itself into a magnificent wreath of soaring snow? First, for the very reason, that it is less in quantity, and more distributed; as a wet cloth smokes when you put it near the fire, but a basin of water not. [Illustration: FIG. 88.] [Illustration: FIG. 89.] [Illustration: FIG. 90. _To face page 128._] The previous heat of the crags, noticed in the first volume, p. 249, is only a part of the cause. It operates only locally, and on remains of sudden showers. But after any number of days and nights of rain, and in all places exposed to returning sunshine and breezes, the _distribution_ of the moisture tells. So soon as the rain has ceased, all water that can run off is of course gone from the steep hill-sides; there remains only the thin adherent film of moisture to be dried; but that film is spread over a complex texture--all manner of crannies, and bosses, and projections, and filaments of moss and lichen, exposing a vast extent of drying surface to the air. And the evaporation is rapid in proportion.