"This vapor which is formed is not supported _by the_ air, as men commonly suppose. It is true that clouds are held up by the atmosphere, but clouds are condensed vapor--minute globules of water floating in the air. Vapor is invisible. You must have noticed that steam is invisible till it is condensed by contact with the colder air. Vapor rests upon the earth and supports itself by its own elastic force, just as the atmosphere supports itself. The presence of air makes no difference with the formation of vapor, except that in a vacuum vapor forms very much more rapidly, because no air stands in its way. But at any given temperature, in the air or in a vacuum, the same amount of vapor rises in due time, and the same amount can support itself. Vapor seems to circulate between the atoms of air, as sand fills the spaces between marbles. At the temperature of four degrees below zero vapor equal to two-thirds of an inch of water can be formed and support itself by its elasticity; that is, the elastic force of vapor at four degrees below zero is equal to two-fifths of an ounce per square inch; at thirty-six degrees vapor equal to two and two-thirds inches of water can support itself; at eighty degrees vapor equal to thirteen inches of water can exist; at one hundred and seventy-nine degrees, seventeen feet; and at two hundred and twelve degrees nearly thirty-four feet; that is, vapor at two hundred and twelve degrees has an elastic force of fifteen pounds to the square inch. Let us suppose that at sunrise the air has a temperature of thirty-six degrees, and that as much vapor is already formed as can sustain itself at that temperature. As the sun sheds down his rays the temperature rises and more vapor is formed. We will suppose that half an inch of water is evaporated. Some of this vapor will be carried by ascending currents of air into the higher regions and condensed into clouds, some will be carried by winds into drier and warmer regions, yet the amount of vapor will increase during the day. We will suppose that during the night the temperature falls again to thirty-six degrees; all the excess of vapor above two inches and two-thirds of water will be condensed and become dew or fog, and in this condensation the thousand degrees of heat absorbed in the formation of the vapor will be given out again. If vapor equal to one inch of water be condensed, heat is set free sufficient to boil a sheet of ice water, five and a half inches in thickness, extending over the whole region; that is, it would be all the same as if a fire were kindled on every square rod of land hot enough to boil during the night more than twenty barrels of ice water. In this illustration I have supposed a larger condensation than commonly takes place, but very much less than is conceivable. Suppose that the temperature is eighty degrees, and that, as is possible, more than one foot of water exists in the state of vapor. Let the temperature fall to