3104
THE NEW ZEALAND GAZETTE.
[No. 74

THE TENTATIVE METHOD OF COMPASS-ADJUSTMENT AS GENERALLY PRACTISED BY COMPASS-ADJUSTERS IN SHIPS OF THE MERCANTILE MARINE.

Before describing the practice, it will be as well to briefly state the coefficients used to express the different magnetic forces:—

Coefficient A represents constant quantity.
,, B ,, semicircular deviation due to fore-and-aft magnetic forces.
,, C ,, semicircular deviation due to transverse magnetic forces.
,, D ,, quadrantal deviation due to horizontal induction in soft iron.
,, E ,, quadrantal deviation due to horizontal induction in soft iron unsymmetrically distributed.

Signs and Effects of Coefficients A, B, C, D, and E.

Coefficient A represents a constant deviation of the same nature and amount on all points of the compass; + A signifying easterly, and — A westerly, deviation.

Coefficient + B represents an attraction towards the ship’s head in the eastern semicircle of the compass, and westerly deviations in the western semicircle, attaining a maximum value on the east and west points, decreasing to zero on north and south points, by compass.

Coefficient — B represents an attraction towards the ship’s stern, and causes easterly deviations with ship’s head in the western semicircle, and westerly deviations with the ship’s head in the eastern semicircle, with a maximum value on the east and west points, decreasing to zero on the north and south points, by compass.

Coefficient + C represents an attraction towards the starboard side of the ship, and causes easterly deviations with ship’s head in the northern semicircle, and westerly deviations in the southern semicircle, attaining a maximum value on the north and south points, decreasing to zero on the east and west points, by compass.

Coefficient — C represents an attraction towards the port side of the ship, and causes westerly deviations with ship’s head in the northern semicircle, and easterly in the southern semicircle, attaining a maximum value on the north and south points, decreasing to zero on the east and west points, by compass.

Coefficient + D gives easterly deviations with ship’s head between N. and E., and S. and W.; and westerly deviations between S. and E., and N. and W.

Coefficient — D gives results exactly the reverse to + D.

NOTE.—Both + D and — D have a maximum value on the four quadrantal points, and become zero on the cardinal points, by compass.

Coefficient + E gives easterly deviations with ship’s head between N.E. and N.W., and S.E. and S.W., and westerly deviations between N.E. and S.E., and N.W. and S.W.

Coefficient — E gives results exactly the reverse to + E.

NOTE.—Both + E and — E have a maximum value on the cardinal points, and become zero on the four quadrantal points, but are usually small in amount in compasses placed in the middle line of the ship.

Heeling Error.—The heeling error arises partly from vertical induction in transverse iron, and iron vertical to the ship’s deck, and partly from the vertical component of the subpermanent magnetism of the ship. In the Northern Hemisphere in the majority of cases the north point of the compass-needle is drawn to windward, or the high side of the ship, with, as a rule, a maximum heeling error on north and south points, and zero on east and west points, by compass. If the compass is not properly placed in the ship there may be a sensible heeling deviation on east and west courses.

Tentative Corrections.

To correct Coefficient C.—With ship’s head north or south, magnetic, place a bar magnet (or more than one if necessary) horizontally and exactly athwart-ship, either on the deck or on any convenient platform, with its centre on the fore-and-aft line passing through the centre of the compass-card, placing its red or marked end to starboard if the north point of the needle deviates to the starboard side, or to port if it deviates to the port side of the ship, moving the magnet to and from the compass until it points correctly.

NOTE.—The deviation represented by coefficient C varies inversely at the earth’s horizontal force, providing the iron is symmetrically arranged on each side of the compass.

To correct Coefficient B.—With ship’s head east or west, magnetic, place a bar magnet (or more than one if necessary) horizontally and exactly parallel to the fore-and-aft midship line of the ship, either on the deck or on any convenient platform, with its centre on the athwartship line passing through the centre of the compass, the red or marked end of the magnet being directed aft if the north point of the compass-needle deviates towards the stern, or forward if it deviates towards the bow, moving the magnet to or from the compass until it points correctly.

NOTE.—The coefficient B consists of two parts: one is due to the permanent magnetism of the ship, which varies inversely as the earth’s horizontal force; the other to vertical induction in soft iron, which varies as the tangent of the dip. As ships in the merchant service are rarely built with a view of providing a satisfactory position for the standard compasses, it is very difficult in many ships to find a position for it where it will not be affected by vertical iron. It follows, therefore, that if this deviation be compensated, as is customary, by a fore-and-aft permanent magnet instead of by a vertical soft-iron bar, the poles of the magnet may in some cases require to be reversed in high southern latitudes.

To correct Coefficient + D.—With ship’s head on one of the quadrantal points, magnetic, the + D is generally corrected by boxes of small chain, cylinders of soft iron, or soft-iron globes, placed athwartships on the same level, and at equal distances, on the port and starboard sides of the compass, with the centre of their mass on a level with the compass-needle, moving them to or from the compass till the needle points correctly.

To correct Coefficient — D.—Coefficient — D, which is of very rare occurrence, is corrected by placing the above correctors on the fore and aft sides of the compass.

NOTE.—When once the coefficient D is properly corrected by soft iron it is correct for all magnetic latitudes, provided the distribution of the iron in the ship is not materially changed, and provided the magnetism of the soft iron has not been affected by the compass-needles. With short needles having small magnetic power, such, for instance, as Sir William Thomson’s, there will probably be no change; but when a compass with long and powerful needles is employed, soft-iron correctors placed very near it become magnetized by induction according to the power of the needles, and the resulting correction will not remain strictly perfect in all latitudes.

To correct Heeling Error.—The heeling error is corrected for any given magnetic latitude by placing a vertical magnet exactly under the centre of the compass-card, with its north or red pole uppermost if the heeling error is to windward, or to the high side of ship, or