The quantity of water thus suspended over the earth is enormous. "When we see a cloud resolve itself into rain, and pour out thousands of gallons of water, we can not comprehend how it can float in the atmosphere."[69] The explanation is--1st, the extreme levity of the minute globules, which causes them to fall very slowly; 2d, they are supported by currents of air, especially by the ascending currents developed both in still air and in storms; 3dly, clouds are often dissolving on one side and forming on another. A cloud gradually descending may be dissolving away by evaporation at the base as fast as new matter is being added above. On the other hand, an ascending warm current of air may be constantly depositing moisture at the base of the cloud, and this may be evaporating under the solar rays above. In this case a cloud is "merely the visible form of an aerial space, in which certain processes are at the moment in equilibrium, and all the particles in a state of upward movement."[70] But so soon as condensation markedly exceeds evaporation, rain falls, and the atmosphere discharges its vast load of water--how vast we may gather from the fact that the waters of all the rivers are but a part of the overflowings of the great atmospheric reservoir. "God binds up the waters in his thick cloud, and the cloud is not rent under them." It is thus that the terrestrial waters are divided into those above and those below that expanse of clear air in which we live and move, exempt from the dense, dark mists of the earth's earlier state, yet enjoying the benefits of the cloudy curtain that veils the burning sun, and of the cloudy reservoirs that drop down rain to nourish every green thing.