ugh within the influence of whatever horizontal movement of air may
be caused by such temporary increase or diminution of pressure. Hence
the barometer shows, and generally foretells, changes of wind; but as
complications always occur, and as changes are of greater or less
extent, affecting or extending through a wider or more limited area,
accompanied by hygrometric and electrical alterations, it is extremely
difficult at times to say beforehand what particular change of weather
is to be expected, and at what interval of time; although after the
event the correspondence of barometric changes with those of the weather
can be readily traced. However, notwithstanding occasional perplexity,
the general character of weather during the next few days may be
predicted by an observer who understands the nature and use of this
instrument and the thermometer, and has watched them in the few
immediately preceding days.

In endeavouring to foretell weather, the general peculiarity should
always be remembered, that the barometric column usually stands higher
with easterly than it does with westerly winds; and with winds from the
polar regions higher than with those from the direction of the equator.
Hence the highest columns are observed with north-east winds in northern
latitudes, and with south-east in the southern hemisphere.

In middle latitudes there is an average difference (unreduced or
observed height as read off) of about half an inch, other things being
similar, between the heights of the mercury with North-easterly, and
with South-westerly winds.

The steadier the column, or the more gradually it moves, the more
settled in character will the weather be, and conversely: because it
shows a quiet settled state of the atmosphere; or, if otherwise, the
reverse. In the tropics, when the barometric column moves contrary to
its usual daily motion, inferior weather may be expected (temporarily),
because the usual air currents are disturbed.

This regular movement, whether tidal, or otherwise connected with the
sun's influence--sensible in tropical latitudes, but more or less masked
elsewhere--amounts to nearly two-tenths of an inch near the equator, the
highest being at about nine, and the lowest near three o'clock.

* * *

Some movements of the atmosphere may be illustrated by reference to the
motion of water drawn off from a reservoir by a small opening below; or
by similar _upward_ draught through a syp