The Hurricane Guide | Page 5

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 courses.
10. Whole gale or that with which she could scarcely bear close-reefed main topsail and reefed foresail. 11. Storm or that which reduces her to storm staysails. 12. Hurricane or that which no canvas could withstand.
Corrections.--As soon after the observations have been made as circumstances will permit, the reading of the barometer should be corrected for the relation existing between the capacities of the tube and cistern (if its construction be such as to require that correction), and for the capillary