The Hurricane Guide | Page 5

connexion with such observations particular
attention should be paid to the direction of the wind according to the
season.
SECTION I.--INSTRUMENTS.

Description and Position of Instruments.--The principal instrument
requisite in these observations is the barometer, which should be of the
marine construction, and as nearly alike as possible to those furnished
to the Antarctic expedition which sailed under the command of Sir
James Clark Ross. These instruments were similar to the ordinary
portable barometers, and differed from them only in the mode of their
suspension and the necessary contraction of the tubes to prevent
oscillation from the motion of the ship. The barometer on shipboard
should be suspended on a gimbal frame, which ought not to swing too
freely, but rather so as to deaden oscillations by some degree of friction.
To the upper portion of the tube in this construction of instrument light
is alike accessible either in front or behind, and the vernier is furnished
with a back and front edge, both being in precisely the same plane,
nearly embracing the tube, and sliding up and down it by the motion of
rack-work; by the graduation of the scale and vernier the altitude of the
mercury can be read off to ·002 inch.
When the barometer is placed in the ship, its position should be as near
midships as possible, out of the reach of sunshine, but in a good light
for reading, and in a situation in which it will be but little liable to
sudden gusts of wind and changes of temperature. Great care should be
taken to ascertain the exact height of its cistern above the water-line,
and in order to facilitate night observations every possible arrangement
should be made for placing behind it a light screened by white paper.
Observations.--The first thing to be done is the reading off and
recording the temperature indicated by the thermometer that in this
construction of instrument dips into the mercury in the cistern. Sir John
Herschel has suggested that "the bulb of the thermometer should be so
situated as to afford the best chance of its indicating the exact mean of
the whole barometric column, that is to say, fifteen inches above the
cistern enclosed within the case of the barometer, nearly in contact with
its tube, and with a stem so long as to be read off at the upper level."
Previous to making an observation with the barometer the instrument
should be slightly tapped to free the mercury from any adhesion to the
glass; any violent oscillation should, however, be carefully avoided.

The vernier should then be adjusted to the upper surface of the mercury
in the tube; for this purpose its back and front edges should be made to
coincide, that is, the eye should be placed in exactly the same plane
which passes through the edges; they should then be brought carefully
down until they form a tangent with the curve produced by the convex
surface of the mercury and the light is just excluded from between them
and the point of contact. It is desirable in making this adjustment that
the eye should be assisted by a magnifying-glass. The reading of the
scale should then be taken and entered in the column appropriated to it
in the proper form. If the instrument have no tubular or double-edged
index, the eye should be placed carefully at the level of the upper
surface of the mercury and the index of the vernier brought gently
down to the same level so as apparently just to touch the surface, great
care being taken that the eye index and surface of the mercury are all in
the same plane.
Each observation of the barometer should be accompanied by an
observation of the direction of the wind, which should be noted in the
usual manner in which it is observed at sea. In connexion with the
direction the force of the wind should be recorded in accordance with
the following scale, contrived by Admiral Sir Francis Beaufort:--
0. Calm 1. Light air or just sufficient to give steerage way. 2. Light
breeze { or that in which a well- } 1 to 2 knots. 3. Gentle breeze
{ conditioned man of war, } 3 to 4 knots. 4. Moderate breeze { with all
sail set, and } 5 to 6 knots. { clean full, would go in } { smooth water,
from } 5. Fresh breeze } { Royals, &c. 6. Strong breeze }
{ Single-reefed top-sails } { and top-gallant } or that in which such a
{ sails. 7. Moderate gale } ship could just carry in { Double-reefed }
chase full and by { topsails, jib, &c. 8. Fresh gale } { Triple-reefed }
{ topsails, &c. 9. Strong gale } { Close-reefed top-sails } { and