In the Northern Hemisphere, when the Polar and Equatorial currents of
air are alternating with each other, the regular shifting of the wind
round the compass will be through S., S.W., W., N.W., N., N.E., E.,
S.E., S.—and the changes will take place oftener between south and
west, and between north and east, than between west and north, and
between east and south.
Similarly in the Southern Hemisphere, it may be inferred that when
Polar currents of air alternate with Equatorial, the regular shifting
of the wind round the compass will be successive through S., S.E., E.,
N.E., N., N.W., W., S.W., S.
This is Dove’s “Law of Gyration,” (or the law of the rotation of
winds). And collecting the main points into a few general propositions,
it may be laid down, that all steady winds are modified by the rotation
of the earth, in such a manner that Equatorial currents of air receive
a westerly deflection, and Polar currents an easterly deflection; but
latitudinal currents suffer no change. The N.E. and S.E. Trade Winds
are steady Polar currents. The Monsoons are alternations of a Polar and
an Equatorial current, twice during the year. Therefore they are N.E.
and S.W. in the Northern Hemisphere, and S.E. and N.W. in the Southern
Hemisphere.
The S.W. passage winds of the Northern Hemisphere, and the N.W. passage
winds of the Southern Hemisphere, are Equatorial currents.
Bodies of air set in motion from a state of rest, turn the wind-vane in
the direction of the meridian as follows:
The Polar current in the Northern Hemisphere from N. to E.
The Polar current in the Southern Hemisphere from S. to E.
The Equatorial current in the Northern Hemisphere from S. to W.
The Equatorial current in the Southern Hemisphere from N. to W.
In general the winds in the Northern Hemisphere are as follows:
Those from N. to E. the Polar currents.
Those from E. to S. the transition of the Polar to the Equatorial.
Those from S. to W. the Equatorial current.
Those from W. to N. the transition of the Equatorial to the Polar.
Also, in the Southern hemisphere, the winds are as follows:
Those from S. to E. the Polar currents.
Those from E. to N. the transition of the Polar to the Equatorial.
Those from N. to W. the Equatorial current.
Those from E. to S. the transition of the Equatorial to the Polar.
Thus, a complete revolution of the wind-vane in the Northern Hemisphere
is therefore S.W., N.E., S. with the sun, and in the Southern Hemisphere
the rotation is S.E., N.W., S., with the sun, also, in that region.
When the course of a steady wind is obstructed in such a manner as to
produce a Hurricane, or Cyclone, the wind has a rotary or whirling
motion (as it were, on an axis), while the storm itself has a
progressive motion.
N. B.—The rotation of the wind during a hurricane, in the Northern
Hemisphere, is in a direction contrary to that in which the hands of a
watch move. In the Southern Hemisphere, the rotation is in the same
direction as that of the hands of a watch. And hence the following
shiftings of the vane: In the Northern Hemisphere, when the vortex
(or center) of the storm passes to the westward of the place of
observation, the rotation is S.W., N.E., S., with the sun; when the
vortex passes to the eastward of the place of observation, the rotation
is S.E., N.W., S., against the sun.
In the Southern Hemisphere, if the vortex passes to the westward of the
place of observation, the rotation is S.E., N.W., S., with the sun.
If the vortex passes to the eastward of the place of observation, the
rotation is S.W., N.E., S., against the sun.
On Hurricanes or Cyclones.
Thus, in both Hemispheres, the passage of an equatorial whirlwind
storm on the west side of the place of observation produces normal
revolutions in accordance with the law of gyration; while, on the other
hand, its passage on the east side produces anomalous revolutions
contrary to the law of gyration. When the vortex of a whirlwind storm
passes over the place of observation, the vane may shift from one
direction to that immediately opposite to it.
Generally, if the whirlwind storm is an equatorial one in the Northern
Hemisphere, it progresses from S.E. to N.W. in the Torrid Zone; it
recurves at the outer edge of the N.E. trade wind, and thence moves
from S.W. to N.E. But in the Southern Hemisphere an equatorial storm
moves first from N.E. to S.W.; recurves at the outer edge of the S.E.
trade wind, and then advances from N.W. to S.E.—bearing in mind
the direction of the rotation of the wind in each hemisphere, viz:
against watch hands in the Northern, but with watch hands in the
Southern Hemisphere. Certain portions of the storms are characterized
by certain hurricane winds, and dividing the storm by diameters drawn
from the northern to the southern margin, and again from the eastern
to the western margin, we find that in the Northern Hemisphere, on the
northern margin, the wind will be easterly; on the eastern margin,
southerly; on the southern margin, westerly; and on the western margin,
northerly;—each portion of the cyclone possessing its appropriate wind.
The relations of the winds to the margin in the Southern Hemisphere,
will be exactly the reverse of their relation in the Northern. Thus: It
is the southern margin of the storm south of the Equator, that exhibits
an easterly; the western margin a southerly; the northern margin a
westerly, and the eastern margin a northerly wind. Hence, each portion
of the hurricane having its appropriate wind, there results, according
to the law of the rotation of revolving gales, a very simple rule for
determining the bearing of the center of the storm from the ship, viz:
Look to the wind’s eye and set its bearing by compass, the _eighth_
point to the right thereof when in the Northern Hemisphere; but to the
left of the wind’s direction when in the Southern Hemisphere, will be
the bearing of the storm’s center. Thus, in the Northern Hemisphere,
from an easterly wind, which characterizes the northern margin of the
storm, its center will bear south; from a northerly wind the center
will bear east; from a westerly wind it will bear north, and from a
southerly wind, west. But in the Southern Hemisphere, from an easterly
wind, the center of the storm bears north; from a southerly wind the
center bears east; from a westerly wind, south; and from a northerly
wind, west.
The above rule is perfectly clear and definite; but as it is especially
important to avoid the center of the storm,—its most dangerous
part,—the following table shows at a glance its relative bearing in
each Hemisphere.
═════════════════════════════════╦══════════════════════════════════
IN THE NORTHERN HEMISPHERE. ║ IN THE SOUTHERN HEMISPHERE.
─────────────┬───────────────────╫─────────────┬────────────────────
If the wind │ The center of the ║ If the wind │ The center of the
be— │ storm will bear ║ be— │ storm will bear
│ from the ship— ║ │ from the ship—
─────────────┼───────────────────╫─────────────┼────────────────────
North. │ East. ║ North. │ West.
N by E │ E by S ║ N by E │ W by N
NNE │ ESE ║ NNE │ WNW
NE by N │ SE by E ║ NE by N │ NW by W
NE │ SE ║ NE │ NW
NE by E │ SE by S ║ NE by E │ NW by N
ENE │ SSE ║ ENE │ NNW
E by N │ S by E ║ E by N │ N by W
East. │ South. ║ East. │ North.
E by S │ S by W ║ E by S │ N by E
ESE │ SSW ║ ESE │ NNE
SE by E │ SW by S ║ SE by E │ NE by N
SE │ SW ║ SE │ NE
SE by S │ SW by W ║ SE by S │ NE by E
SSE │ WSW ║ SSE │ ENE
S by E │ W by S ║ S by E │ E by N
South. │ West. ║ South. │ East.
S by W │ W by N ║ S by W │ E by S
SSW │ WNW ║ SSW │ ESE
SW by S │ NW by W ║ SW by S │ SE by E
SW │ NW ║ SW │ SE
SW by W │ NW by N ║ SW by W │ SE by S
WSW │ NNW ║ WSW │ SSE
W by S │ N by W ║ W by S │ S by E
West. │ North. ║ West. │ South.
W by N │ N by E ║ W by N │ S by W
WNW │ NNE ║ WNW │ SSW
NW by W │ NE by N ║ NW by W │ SW by S
NW │ NE ║ NW │ SW
NW by N │ NE by E ║ NW by N │ SW by W
NNW │ ENE ║ NNW │ WSW
N by W │ E by N ║ N by W │ W by S
─────────────┴───────────────────╨─────────────┴────────────────────
The path which the axis of rotation describes is not inappropriately
termed the “Axis Line.” On the “Axis Line” there is no change of wind
until the center has passed, when, after a brief interval of calm, the
wind springs up with great fury from the opposite quarter.
It has been remarked by Piddington, “that he who watches his barometer,
watches his ship.” This invaluable instrument, if well understood,
invariably announces the approach of a revolving storm; shows whether
the vessel is plunging into the vortex, or if she be receding from
it; and hence, by carefully noticing its indications, the disastrous
consequences of a hurricane may, to a great extent, be avoided,—for
the laws of its oscillations are very distinctly marked. The barometer
often stands unusually high before the commencement of a cyclone, and
frequently (if not always) just around the storm. And conceiving the
cyclone to be divided into two parts by a diameter at right angles
to its path, it may be noted that the barometer always falls during
the passage of the advancing semi-circle of a revolving storm. The
barometer always rises during the passage of the receding semi-circle
of a revolving storm. In cases of manɶuvering to take advantage of the
hurricane winds, and to keep just within the verge of the storm, the
barometer is of signal benefit,—it should be kept as high as possible,
without losing the wind. In whatever position the ship may be, the
rising of the mercury announces that the first (or dangerous) half of
the storm has passed.
The barometer is a faithful guide in the zone of the Trade Winds.
Its ordinary variations are so small that any deviation from its
normal height requires attention. Its fluctuations are larger beyond
the Tropics; but the indications are scarcely less certain if used
in conjunction with the thermometer and hygrometer,—hence, more
watchfulness is required on the part of the navigator; and it is of the
utmost importance to have such a knowledge of the Law of Storms, of
storm instruments, and of the signs of the weather, as to be able to
detect the presence of a hurricane or cyclone, when as yet sky and sea
betoken but little of that blind fury, during the height of which it is
so difficult to guide the vessel in safety. One great advantage of such
knowledge is a quick apprehension of the indications which forewarn, to
make all snug in time.
The meteorological signs indicating the approach of a hurricane or
cyclone, have been collected from various sources by Mr. Birt, and are
enumerated and classed according as the phenomena may be presented to
the external sensations of feeling, sight, or hearing.