Now the pressure of the latter is measured by the barometrical column, about 30 inches of mercury, while that of watery vapor is equal to one inch of mercury at the constituent temperature of 80 degrees, and to one fifth of an inch at the temperature of 32 degrees. If the atmosphere of air by which we are supported were annihilated, there would still remain, an atmosphere of aqueous vapor, arising from the waters and moist parts of the earth, but in the existing state of things this vapor rises in the atmosphere of dry air, and thus its distribution and effects are materially influenced by the vehicle in which it is thus carried. The moisture thus floating at all times in the air, serves for the support of vegetable life, even in countries where rain seldom if ever falls. It is absorbed by the leaves of living plants, which thus increase in weight even when suspended in

the atmosphere and disconnected with the soil. During intense heats, and when the soil is parched and dry, we see the life of plants thus preserved until the earth is

again refreshed with showers, and the roots supplied with their wonted moisture. Clouds, are produced when aqueous vapor returns to the state of water ; and this process is called condensation. Whenever the temperature becomes lower than the constituent temperature, requisite for the maintenance of the vapory state, some

of the vapor, or invisible steam, will be condensed, and become water. This may be seen illustrated in the condensation of the steam, as it issues from the spout of a tea-kettle. Clouds not only moderate the fervor of the sun, but they also check radiation from the earth, for we find that the coldest nights are those which occur under a cloudless winter sky.