ot,
perhaps, under all the current drawn towards it; and the other, a
consequence of opposing winds, which occasion huge eddies or whirlwinds
of compression.

Some whirlwinds are accompanied by rushes from the upper atmosphere,
from the colder regions, which, mingling with warmer and moister air
near the sea, cause dense clouds. About their centre it sometimes
happens that the barometer falls as much as two or three inches, showing
a diminution of atmospheric pressure by nearly a tenth part; when it
should be expected, from physical considerations alone, that very dense
clouds would be formed.[24]

* * *

The column of mercury falls about one tenth of an inch for each of the
first few hundred feet above the sea level, but varying when it becomes
much more elevated.[25] Due allowance, therefore, should be made in
observing, when on high land.

The tides are affected by atmospheric pressure, so much that a rise of
one inch in the barometer will have a corresponding fall in the tides of
nine to sixteen inches, or about one foot for each inch.[26]

* * *

Vessels sometimes enter docks, or even harbours, where they have
scarcely a foot of water more than their draught; and as docking, as
well as launching large ships, requires a close calculation of height of
water, the state of the barometer becomes of additional importance on
such occasions.

* * * * *

To render these pages rather more useful at sea, in _any_ part of the
world, a few words about squalls and hurricanes are here offered to the
young seaman.

Generally, squalls are preceded, or accompanied, or followed by clouds;
but the very dangerous "white squall" (of the West Indies and other
regions), is indicated only by a rushing sound, and by white wave
crests.

"Descending squalls" come slanting downwards, off high land,[27] or from
upper regions of atmosphere. They are dangerous, being sometimes
violently strong.

A squall cloud that can be seen through or under is not likely to bring,
or be accompanied by, so much wind as a dark continued cloud extending
beyond the horizon. How the comparative hardness or softness of clouds
foretells more or less wind or rain, was stated in pages 13 and 14.

The expressions "hardening up," "softening," or looking "greasy," are
familiar to seamen: and such very sure indications are the appearances
so designated, that they can hardly be mistaken.