of the principles involved.

The earth is heated by radiation from the sun. The air is not warmed
by the passage of the sun's heat through it, but by convection from
the earth, in the same way that it is warmed by the surfaces of stoves.
The lower stratum of air is warmed by the earth and by objects which
have been warmed by radiated heat from the sun. The particles of air
thus heated expand, become lighter, and rise, being replaced by the
descent of the cooler and heavier particles from above, which, on being
warmed also rise, and give place to others. Owing to this process, the
air is warmest nearest the earth, and grows cooler as height increases.

The air has a strong attraction for water, and always holds a certain
quantity as invisible vapor. The warmer the air, the more moisture it
demands, and it will draw it from all objects within reach. The air
holds water according to its temperature. Thus, at fifty-two degrees,
Fahrenheit's thermometer, it holds half the moisture it can sustain;
but at thirty-six degrees, it will hold only one eighty-sixth part.
The earth and all plants and trees are constantly sending out moisture;
and when the air has received all it can hold, without depositing it
as dew, it is said to be _saturated_, and the point of temperature
at which dew begins to form, by condensation, upon the surface of the
earth and its vegetation, is called the _dew-point_. When air,
at a given temperature, has only forty per cent of the moisture it
requires for saturation, it is said to be dry. In a hot summer day,
the air will hold far more moisture than in cool days. In summer,
out-door air rarely holds less than half its volume of water. In 1838,
at Cambridge, Massachusetts, and New-Haven, Connecticut, at seventy
degrees, Fahrenheit, the air held eighty per cent of moisture.