1492
THE NEW ZEALAND GAZETTE.
[No. 43

Tentative Corrections.

To correct Co-efficient C.—With ship’s head north or south, magnetic, place a bar magnet (or more than one if necessary) horizontally and exactly athwartship, 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 or from the compass until it points correctly.

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

To correct Co-efficient 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 co-efficient 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 Co-efficient + 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 Co-efficient — D. — Co-efficient — 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 co-efficient 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 becomes 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 its south or blue end uppermost if to leeward or to the low side of ship, moving the magnet to or from the compass until the heeling error is corrected.

NOTE.—The heeling error due to the permanent part of the magnetism varies inversely as the earth’s horizontal force, and consequently is greatest in high latitudes, diminishes as the ship approaches the magnetic equator, and increases again, still retaining the same name, as the ship recedes from the magnetic equator in the Southern Hemisphere. The heeling error due to transverse and vertical soft iron decreases as the ship approaches the magnetic equator, where it is zero, and is of a contrary name in the Southern Hemisphere. It is probable that the poles of the vertical magnet may require to be reversed in high southern latitudes.

A divided scale should be marked or fitted outside the tube or some other convenient place, so as to show the proper position for the correcting magnet as found in any given magnetic latitude, and the same recorded as a guide for approximately placing the magnet in position on any subsequent voyage in the same locality, and especially on the return of the ship to the United Kingdom.

Candidates should understand that the object of tentative adjustment is to bring the deviations within manageable limits, and also to equalise the directive force of the needle so far as is practicable on all courses; but no system of adjustment whatever is sufficiently reliable in character to absolve the navigator from the necessity of using every precaution, and especially of ascertaining the deviation on every available opportunity by observations of the sun by day and the other heavenly bodies by night.

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APPENDIX N.

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EXAMINATION OF A MASTER OR MATE IN STEAM.

THE regulations under which these examinations are conducted are printed at pages 37 to 39.

A candidate for this examination is required to have a thorough grasp of the construction of the steam-engine and boiler, to enable him to understand the nature and importance of any defect which may be reported to him by the engineer, and work in harmony with the engineer in affording time and facilities for disconnections, inspections, adjustments, and repairs.

To have a knowledge of what the principal repairs are which are needed in engines and boilers and pipes, and how these repairs are usually accomplished.

To be able to form an independent opinion as to a breakdown, and the consequent propriety or impropriety of proceeding under reduced steam with temporarily repaired or defective machinery.

To understand how to estimate approximately the reduction of fuel required for reduced speed, and