It is well known that a cold pitcher in summer robs the vapor in the atmosphere of its heat, and causes it to be deposited on its own surface. It looks as though the pitcher were _sweating_, but the water all comes from the atmosphere, not, of course, through the sides of the pitcher. If we breathe on a knife-blade, it condenses in the same manner the moisture of the breath, and becomes covered with a film of water. Stone houses are damp in summer, because the inner surfaces of the walls, being cooler than the atmosphere, cause its moisture to be deposited in the manner described. By leaving a space, however, between the walls and the plaster, this moisture is prevented from being troublesome. [How does this principle affect the soil? Explain the experiment with the two boxes of soil.] Nearly every night in the summer season, the cold earth receives moisture from the atmosphere in the form of dew. A cabbage, which at night is very cold, condenses water to the amount of a gill or more. The same operation takes place in the soil. When the air is allowed to circulate among its lower and _cooler_ particles, they receive moisture from the same process of condensation. Therefore, when, by the aid of under-drains, the lower soil becomes sufficiently open to admit of a circulation of air, the deposit of atmospheric moisture will keep the soil supplied with water at a point easily accessible to the roots of plants. If we wish to satisfy ourselves that this is _practically_ correct, we have only to prepare two boxes of finely pulverized soil, one, five or six inches deep, and the other fifteen or twenty inches deep, and place them in the sun at mid-day in summer. The thinner soil will be completely dried, while the deeper one, though it may have been perfectly dry at first, will soon accumulate a large amount of water on those particles which, being lower and more sheltered from the sun's heat than the particles of the thin soil, are made cooler. With an open condition of subsoil, then, such as may be secured by under-draining, we entirely overcome drought. [How does under-draining supply to the soil an increased amount of atmospheric fertilizers? How does it warm the lower parts of the soil?] 2. Under-draining _furnishes an increased supply of atmospheric fertilizers_, because it secures a change of air in the soil. This change is produced whenever the soil becomes filled with water, and then dried; when the air above the earth is in rapid motion, and when the comparative temperature of the upper and lower soils changes. It causes new quantities of the ammonia and carbonic acid which it contains to be presented to the absorbent parts of the soil. 3. Under-draining _warms the lower parts of the soil_, because the deposit of moisture (1) is necessarily accompanied by an abstraction of heat from the atmospheric vapor, and because heat is withdrawn from the whole amount of air circulating through the cooler soil.