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 be— | The center of the storm will bear from the ship— | If the wind be— | The center of the storm will bear from the ship— |
| North. | East. | North. | West. |
| N by E | E by S | N by E | W by N |
| N N E | E S E | NNE | WNW |
| NE by N | SE by E | NE byN | 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 | N W | 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.