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.
In New Orleans, the air often retains ninety per cent of the moisture
it is capable of holding; and in cool days at the North, in foggy
weather, the air is sometimes wholly saturated.
When air holds all the moisture it can, without depositing dew, its
moisture is called 100. When it holds three fourths of this, it is
said to be at seventy-five per cent. When it holds only one half, it
is at fifty per cent.
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