Changeable_, and then rise towards _Fair_, it presages a
change of wind or weather, though not so great, as if the mercury had
risen higher; and, on the contrary, if the mercury stand above _fair_
and then fall, it presages a change, though not to so great a degree as
if it had stood lower: besides which, the direction, and force of wind,
are not in any way noticed. It is not from the point at which the
mercury may stand that we are alone to form a judgment of the state of
the weather, but from its _rising_ or _falling_; and from the movements
of immediately preceding days as well as hours, keeping in mind effects
of change of _direction_, and dryness, or moisture, as well as
alteration of force or strength of wind.

* * *

In this part of the world, towards the higher latitudes, the quicksilver
ranges, or rises and falls, nearly three inches--namely, between about
thirty inches and nine-tenths (30.9), and less than twenty-eight inches
(28.0) on extraordinary occasions; but the usual range is from about
thirty inches and a half (30.5), to about twenty-nine inches. Near the
Line, or in equatorial places, the range is but a few tenths, except in
storms, when it sometimes falls to twenty-seven inches.

The sliding-scale (vernier) divides the tenths into ten parts each, or
hundredths of an inch. The number of divisions on the vernier exceeds
that in an equal space of the fixed scale by one.[7]

* * *

By a thermometer the _weight_ of air is _not_ shown. No air is within
the tube. None can get in. But the bulb of the tube is full of mercury,
which contracts by cold, and swells by heat--according to which effect
the thread of metal in the small tube is drawn down or pushed up so many
degrees: and thus shows the temperature.[8]

If a thermometer have a piece of linen tied round the bulb, wetted
enough to keep it damp by a thread or wick dipping into a cup of water,
it will show less heat than a dry one, in proportion to the dryness of
the air, and quickness of drying.[9] In very damp weather, with or
_before_ rain, fog, or dew, two such thermometers will be nearly alike.

For ascertaining the dryness or moisture of air, the readiest, and
surest method is the comparison of two thermometers; one dry, the other
_just_ moistened, and _kept so_. Cooled by evaporation as much as the
state of the air admits--the moist (or wet) bulb thermometer shows a
temperature nearly equal to that of th