First, on their size; that is to say, on the proportion of the magnitude of the object (irrespective of the distribution of its particles) to the magnitude of the particles of the air.
Thus, a grain of sand is buoyant in wind, but a large stone is not; and pebbles and sand are buoyant in water in proportion to their smallness, fine dust taking long to sink, while a large stone sinks at once. Thus, we see that water may be arranged in drops of any magnitude, from the largest rain-drop, about the size of a large pea, to an atom so small as not to be separately visible, the smallest rain passing gradually into mist. Of these drops of different sizes (supposing the strength of the wind the same), the largest fall fastest, the smaller drops are more buoyant, and the small misty rain floats about like a cloud, as often up as down, so that an umbrella is useless in it; though in a heavy thunder-storm, if there is no wind, one may stand gathered up under an umbrella without a drop touching the feet.
Secondly, buoyancy depends on the amount of surface which a given weight of the substance exposes to the resistance of the substance it floats in. Thus, gold-leaf is in a high degree buoyant, while the same quantity of gold in a compact grain would fall like a shot; and a feather is buoyant, though the same quantity of animal matter in a compact form would be as heavy as a little stone. A slate blows far from a house-top, while a brick falls vertically, or nearly so.
[2] There is a beautiful passage in Sartor Resartus concerning this old Hebrew scroll, in its deeper meanings, and the child’s watching it, though long illegible for him, yet “with an eye to the gilding.” It signifies in a word or two nearly all that is to be said about clouds.
CHAPTER II.
THE CLOUD-FLOCKS.
§ 1. From the tenor of the foregoing chapter, the reader will, I hope, be prepared to find me, though dogmatic (it is said) upon some occasions, anything rather than dogmatic respecting clouds. I will assume nothing concerning them, beyond the simple fact, that as a floating sediment forms in a saturated liquid, vapor forms in the body of the air; and all that I want the reader to be clear about in the outset is that this vapor floats in and with the wind (as, if you throw any thick coloring matter into a river, it floats with the stream), and that it is not blown before a denser volume of the wind, as a fleece of wool would be.
§ 2. At whatever height they form, clouds may be broadly considered as of two species only, massive and striated. I cannot find a better word than massive, though it is not a good one, for I mean it only to signify a fleecy arrangement in which no lines are visible. The fleece may be so bright as to look like flying thistle-down, or so diffused as to show no visible outline at all. Still if it is all of one common texture, like a handful of wool, or a wreath of smoke, I call it massive.