arily afford to lose.

Evaporation is a slow process occurring at all times; it is hastened
during the summer, because of the large amount of heat present in the
atmosphere. Many large cities make use of the cooling effect of
evaporation to lower the temperature of the air in summer; streets are
sprinkled not only to lay the dust, but in order that the surrounding
air may be cooled by the evaporation of the water.

Some thrifty housewives economize by utilizing the cooling effects of
evaporation. Butter, cheese, and other foods sensitive to heat are
placed in porous vessels wrapped in wet cloths. Rapid evaporation of
the water from the wet cloths keeps the contents of the jars cool, and
that without expense other than the muscular energy needed for wetting
the cloths frequently.

27. Rain, Snow, Frost, Dew. The heat of the sun causes constant
evaporation of the waters of oceans, rivers, streams, and marshes, and
the water vapor set free by evaporation passes into the air, which
becomes charged with vapor or is said to be humid. Constant, unceasing
evaporation of our lakes, streams, and pools would mean a steady
decrease in the supply of water available for daily use, if the
escaped water were all retained by the atmosphere and lost to the
earth. But although the escaped vapor mingles with the atmosphere,
hovering near the earth's surface, or rising far above the level of
the mountains, it does not remain there permanently. When this vapor
meets a cold wind or is chilled in any way, condensation takes place,
and a mass of tiny drops of water or of small particles of snow is
formed. When these drops or particles become large enough, they fall
to the earth as rain or snow, and in this way the earth is compensated
for the great loss of moisture due to evaporation. Fog is formed when
vapor condenses near the surface of the earth, and when the drops are
so small that they do not fall but hover in the air, the fog is said
"not to lift" or "not to clear."

If ice water is poured into a glass, a mist will form on the outside
of the glass. This is because the water vapor in the air becomes
chilled by contact with the glass and condenses. Often leaves and
grass and sidewalks are so cold that the water vapor in the atmosphere
condenses on them, and we say a heavy dew has formed. If the
temperature of the air falls to the freezing point while the dew is
forming, the vapor is frozen and frost is seen instead of dew.

The daily