Rising upward from the sea, the vapour of water commonly remains transparent and invisible until it attains a considerable height above the surface, where the cooling tends to make it assume again the visible state of cloud particles. The formation of these cloud particles is now believed to depend on the fact that the air is full of small dust motes, exceedingly small bits of matter derived from the many actions which tend to bring comminuted solid matter into the air, as, for instance, the combustion of meteoric stones, which are greatly heated by friction in their swift course through the air, the ejections of volcanoes, the smoke of forest and other fires, etc. These tiny bits, floating in the air, because of their solid nature radiate their heat, cool the air which lies against them, and thereby precipitate the water in the manner of dew, exactly as do the leaves and other structures on the surface of the earth. In fact, dew formation is essentially like cloud formation, except that in the one case the water is gathered on fixed bodies, and in the other on floating objects. Each little dust raft with its cargo of condensed water tends, of course, to fall downward toward the earth's surface, and, except for the winds which may blow upward, does so fall, though with exceeding slowness. Its rate of descent may be only a few feet a day. It was falling before it took on the load of water; it will fall a little more rapidly with the added burden, but even in a still air it might be months or years before it would come to the ground. The reason for this slow descent may not at first sight be plain, though a little consideration will make it so. If we take a shot of small size and a feather of the same weight, we readily note that their rate of falling through the air may vary in the proportion of ten to one or more. It is easy to conceive that this difference is due to the very much less friction which the smaller body encounters in its motion by the particles of air. With this point in mind, the student should observe that the surface presented by solid bodies in relation to their solid contents is the greater the smaller the diameter. A rough, though not very satisfactory, instance of this principle may be had by comparing the surface and interior contents of two boxes, one ten feet square and the other one foot square. The larger has six hundred feet of surface to one thousand cubic feet of interior, or about half a square foot of outer surface to the cubic foot of contents; while the smaller box has six feet of surface for the single cubic foot of interior, or about ten times the proportion of exterior to contents. The result is that the smaller particles encounter more friction in moving toward the earth, until, in the case of finely divided matter, such as the particles of carbon in the smoke from an ordinary fire, the rate of down-falling may be so small as to have little effect in the turbulent conditions of atmospheric motion.