The supply of rain turns on this ratio, on the volume of the air, on the ratio of water to land, and on changes of temperature. The capacity of the air for vapor increases and diminishes with temperature. A cool wind, therefore, stealing through the atmosphere, will pour down a plentiful shower; yet the coldest winds can not drive all the vapor from the air.

A large amount of heat is consumed in the passage of ice into water and of water into steam. In consequence of this fact ice and snow melt slowly even in a high temperature, and steam forms slowly. By the one circumstance we are saved from deluges and by the other from explosions. These are also facts of great significance in the diffusion of heat. Evaporation on the surface of the earth takes up the heat that occasions it and bears it off in a latent form, while the precipitation of rain in the atmosphere liberates the heat, and so reduces the cold that occasions the shower. A summer rain, or even a winter snow-storm, may be observable for its moderate, uniform temperature. Even the hail modifies the fierce blast that produces it. The snow that filters out of the warming air gives the best of blankets to the frozen earth. The evaporation by day softens the intensity of the sun's heat, while the vapor by night checks the radiation of the earth, and so helps to hold the two extremes of midday and midnight in a grateful equipoise. The simple dryness of a climate subjects it to great extremes of heat and cold. The land that nature does not water, she warms capriciously. Few