Monk, Mason, de Saussure, and many others fix the prime condition for the formation of rain in the superposition of two beds of cloud. This assertion, although it is still repeated in a number of treatises on physics, is inexact. A single stratum of cloud--yes, a solitary cloud--has been seen, on the Puy-de-Dôme, to produce rain and lightning, with thunder. Frequently, under the influence of the centers of perturbation which often exist south of the Alps, a vast sea of clouds, the upper face of which does not exceed an altitude varying from seven hundred to twelve hundred metres, covers all central France, and probably other countries. Only the high table-lands and mountains rise above this stratum of clouds over which the sun shines in a perfectly clear sky. Yet rain is found in such strata of clouds, however homogeneous they may be, and it rains in the regions they cover. I have long been able to affirm this fact, important because it destroys old errors elaborated in the isolation of the study, and to support it with authentic proof. We may witness the formation of rain when we rise into the usual region of the clouds, either in balloon ascents or by climbing mountains. The phenomenon may be observed under five aspects: First, we may find ourselves in a fog of greater or less thickness, the hygrometer indicating that the air is nearly saturated with vapor, without one being able to detect the fall of the smallest liquid particle, and without exterior objects being moistened. Second, while we can not observe the fall of a single liquid drop, however small, everything enveloped in the cloud will be rapidly moistened. We are in the atmospheric stratum where the rain is beginning to form. Inhabitants of mountainous regions say at such times that there is a wet fog. At the top of the Puy-de-Dôme, when this condition lasts for a day, we can collect three, four, or five millimetres of water. Third, we may remark, in the fog, the fall of exceedingly fine droplets, which we can hardly distinguish--it is drizzling. Fourth, the rain is falling, while we are still in the fog; and, fifth, the rain is falling and we are below the fog--that is, below the clouds.