The Columbia herald. [volume] (Columbia, Tenn.) 1850-1873, 1873-05-09

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The secondary cause of a storm is
reduced air-pressure over any particular
part of the earth's service which is in
dicated by depression of the mercury
in the barometer within that region.
Hence the central district of storm is
technically called can era of low bar
ometer" Towards this region the sur
rounding air rushes in to restore an
equilibrium, and that rush of ais is
known as wind. The resulting circular
motion, around the central point, is due
to the rotation of the earth upon her
axis, from west to east.
It is easy to calculate that the velocity
of rotation at the equator, "a, 900 miles
in "a hours, is 17.3 miles per minute,
while at the pole it is nothing. In the
latitude of 40 degrees it is 49, miles;
in the latitude of 45 degrees it is 124
miles per minute. The earth carries the
atmosphere along with it, and the yea
locity of the atmosphere at any point
is the velocity of that point, which de-
creases with an increase in the latitude,
It is evident that, if mass of air at any
point be moved northward of south
ward, from any cause, it will preserve
for time the eastward velocity of the
point from which it started; just as a
stone thrown from the window of a
moving railroad car is carried forward by
the impetus of the train. Hence a mass
of air moving southwark, in this hemis-
phere, will lag behind in the rotation,
while the relative motion will be faster
than that of the surface if it move north
ward, because of the change in velocity
of the surface due to the latitude
Now suppose the top line of our dia-
gam to represent the parallel of As de-
grees of latitude, and the Length of the
horizontal arrow indicate the motion per
minute on the PARALLEL. Let the lower
line represent the parallel of 40 degrees;
then the length of the arrow near it will
indicate the velocity of the air there per
minute. In each case the surface, and
it's superintendent air, moves in the di-
rection of the arrow. If an area of low
barometer be at @, nearly midway be-
tween these two parallels, the air will
move towards that area from every point
of the compass; and the result is indi-
Gated in the diagram, where the dotted
lines show the tendency, and the full
lines show the direction, of the actual
movement. A mass of air coming from
the north at n, moves eastward with
velocity 123 miles per minute, while the
storm-center moves 122 miles per min
ute. Hence, the moving air drags be-
hind at the rate of J mile per minute.
or 30 miles per hour, and takes the dj
rection of the arrow, toward the circum
ference of the small circle, instead of its
center at G. Similarly mass of air NOV
ing northward, from S, has an initial
velocity of 49, miles per hour greater
than that of the center of the storm and
moves in the direction of the arrow from
S, instead of the dotted line. Similarly,
we may show that the same deviation
occurs in the case of the air coming from
every direction toward c.; and the con-
sequence is that the air moves round the
center of the area of low barometer, in
q direction contrary to that of the hands
of a watch that is laid with its face up
ward.
The vertical motion resulting from all
directions towards stOrm-center is even
more decided than indicated by the
diagram, which only represents the
secondary tendency. The central rush
is opposed, or counteracted, by the elias
ITCHY of the several volumes of air as
they approach each other; and their
impetus carries them around in the
direction of the movement already estab-
lished, so that the whirl extends nearly
as far from the center as the distance
from which the winds blow. All the air
that tends towards the storm-center
moves round it, in an approximate circle ;
in Just the same way that water courses
round in funnel, for similar reason,
and in the same direction, the only diff
ference being that the central column of
air moves upward, while the central
column of water in the funnel moves
downward.
A knowledge of these facts enables us
not only to tell the direction of the
storm-center, but often to know whether
it is approaching towards us, or reced-
ing, or is passing US at a harmless dis
lance. If we face the wind the storm
center will always be on the right hand
or if we turn so that the wind blows
from left to right we shall face the
storm-center. If, at the same time, the
mercury in the barometer continues to
fall, we may be sure that the vortex is
approaching US; while, if the barometer
rises. we may count on it's having pass
ed by. The general direction of the
movement of the center is from west to
east; but it may depart several points
from the cardinal line.
The preceding concentrations OF po-
sition apply to the Northern Temper
ate Zone. A little consideration will
help us to understand that, in the
Southern Temperate Zone, the direc-
ton of the rotary movement Is the OPP
site to that of storms in this region
the winds blow round the vortex the
same way that the hands of watch
move. In the Torrid Zone. within what
are technically known " d0ldrums "
(the limits of true cyclone act1on), the
difference between the lengths OF the
successive parallels of latitude is so
small that the conditions of rotary mo-
ton are present but feebly, and not at
all on the equator.
It is evident that this knowledge is
of much greater practical value on the
water than on the land; added to
which, storms move with more regular
ity on the ocean than on shore, the mo-
ton not being interfered with by irreg-
ularity of surface. On land our theo-
retical circle often becomes an irregu-
lar ellipse, but on the ocean the circu-
lar form is very nearly preserved. The
mariner may avoid serious damage, and
perhaps total loss, by a knowledge OF
those facts of storm motion, by steering
as nearly as may be away from the vor-
tex. This may be accomplished by
keeping the starboard side to windward,
and getting out of the track if the
storm-center be moving towards the
first noted place of the vessel. Some OF
our ocean-going seamen are already ed
ueate up to knowledg of these facts,
and an appreciation of their value and
there can be no doubt that more thor
ough dis emination of the principles OF
storm science will result in material
diminutiOn II The annual percentage of