These five aspects may be present in the same cloud, when we will find them in the order given in successive strata, one beneath another; so that, entering such a cloud from the upper part, we may traverse, in regular order, "dry" fog, wet fog, fog with drizzle, fog with rain, and, as we leave the cloud at the bottom, rain without fog. Mr. Glaisher, the English scientific aëronaut, thus records his experience in an ascension he made July 1, 1863: "We let ourselves drop at eight hundred metres, and went into a fog which was dry for the first thirty metres, but shortly afterward became moist. As we descended, the fog seemed to become more charged with water, and seemed very dark beneath us; at five hundred or six hundred metres we heard the sound of the rain striking the trees, so violent was the fall." Rain drops, in fact, grow as they fall, whether by continuance of condensation, or by union with other drops. They should, therefore, be larger when they issue from the cloud in proportion as the region where drizzle is formed is higher above the base of the cloud. There is, however, a limit to the size they can attain, for the velocity of their fall increases with their mass, and they are divided by the resistance of the air. The five aspects under which we have regarded the formation of rain are evidently five phases distinguished by our senses in the progressive transformation which the vapor of water undergoes in passing to the liquid state. It also sometimes happens that the condensation of the vapor in a cloud can only reach the first or second stage of the transformation without extending to the other stages. At other times it stops at the third phase, that of drizzling, which may then, as rain does, cross atmospheric regions below the cloud, and reach the ground, provided the base of the cloud is not too high and the air passed through is not too dry. In short, we may conclude that the formation of rain is due simply to variations in the temperature and moisture of the air. There is, however, another element, the intervention of which is indispensable, if not to reduce the vapor to water, at least to cause that water to fall in rain, or under the form of drops. This element is the atmospheric dust.