180
COMPASS
compass bowl a vertical black line is drawn, called the becomes instantly magnetized by induction when exposed “ lubber’s point,” and it is imperative that when the to any magnetic force, but has no power of retaining iron is less susceptible of being compass is placed in the binnacle the line joining the its magnetism. Hard magnetized, but wThen once magnetized it retains its magnetism permanently. The term “ iron ” used in these pages includes the “ steel ” now commonly employed in shipbuilding. If an iron ship be swung when upright for deviation, and the mean horizontal and vertical magnetic forces at the compass positions be also observed in different parts of the world, mathematical analysis shows that the deviations are caused partly by the permanent magnetism of hard iron, partly by the transient-induced magnetism of soft iron both horizontal and vertical, and in a lesser degree by iron which is neither magnetically hard nor soft, but which becomes magnetized in the same manner as hard iron, though it gradually loses its magnetism on change of conditions, as, for example, in the case of a ship, repaired Fig. 5.—Liquid Compass. A, bowl, partly in section ; B, expansion chamber ; and hammered in dock, steaming in an opposite direction D, the glass ; G, gimbal ring ; L, nut to expand chamber when filling bowl; at sea. This latter cause of deviation is called sub-perW, screw connector ; N, hole for filling, with screw plug ; O, O, magnetic manent magnetism. The horizontal directive force on the needles ; P, buoyant chamber; Q, iridium pivot; R, sapphire cap ; S, mica card. needle on board is nearly always less than on land, sometimes pivot and the lubber’s point be parallel to the keel of the much less, whilst in armour-plated ships it ranges from vessel. Thus, when a degree on the card is observed oppo- ‘8 to '2 when the directive force on land = 1 '0. If the ship site the lubber’s point, the angle between the direction in be inclined to starboard or to port additional deviation will which the ship is steering and the north point of the com- be observed, reaching a maximum on north and south points, pass or course is at once seen; and if the magnetic varia- decreasing to zero on the east and west points. Each ship tion and the disturbing effects of the ship’s iron are known, has its own magnetic character, but there are certain conthe desired angle between the ship’s course and the geo- ditions which are common to vessels of the same type. Instead of observing the deviation solely for the purposes graphical meridian can be computed. In every ship a position is selected for the navigating or standard compass of correcting the indications of the compass when disturbed as free from neighbouring iron as possible, and by this by the iron of the ship, the practice is to subject all compass all courses are shaped and bearings taken. It is deviations to mathematical analysis with a view to their also provided with an azimuth circle or mirror and a mechanical correction. The whole of the deviations when shadow pin or style placed in the centre of the glass cover, the ship is upright may be expressed nearly by five by either of which the variable angle between the compass co-efficients, A, B, C, D, E. Of these A is a deviation north and true north, called the “ total error,” or variation constant in amount for every direction of the ship’s head. and deviation combined, can be observed. The binnacles B has reference to horizontal forces acting in a longior pedestals for compasses are generally constructed of tudinal direction in the ship, and caused partly by the wood about 45 inches high, and fitted to receive and alter permanent magnetism of hard iron, partly by vertical at pleasure the several magnet and soft iron correctors. induction in vertical soft iron either before or abaft the They are also fitted with different forms of suspension in compass. C has reference to forces acting in a transverse which the compass is mounted to obviate the mechanical direction, and caused by hard iron. D is due to transient disturbance of the card caused by the vibration of the hull induction in horizontal soft iron, the direction of which passes continuously under or over the compass. E is due in ships driven by powerful engines. The effects of the iron and steel used in the construction to transient induction in horizontal soft iron unsymmetriof ships upon the compass have occupied the attention of cally placed with regard to the compass. When data of the ablest physicists of the 19th century, with results which this character have been obtained the compass deviations enable navigators to conduct their ships with perfect may be mechanically corrected to within 1°—always adhersafety. The hull of an iron or steel ship is a magnet, ing to the principle that “ like cures like.” Thus the part and the distribution of its magnetism depends upon the of B caused by the permanent magnetism of hard iron direction of the ship’s head when building, this result must be corrected by permanent magnets horizontally being produced by induction from the earth’s magnetism, placed in a fore and aft direction; the other part caused developed and impressed by the hammering of the plates by vertical soft iron by means of bars of vertical soft and frames during the process of building. The disturb- iron, called Flinders bars, before or abaft the compass. ance of the compass by the magnetism of the hull is C is compensated by permanent magnets athwartships and generally modified, sometimes favourably, more often horizontal; D by masses of soft iron on both sides of the unfavourably, by the magnetized fittings of the ship, such compass, and generally in the form of cast-iron spheres, as masts, conning towers, deck houses, engines, and boilers. with their centres in the same horizontal plane as the Thus in every ship the compass needle is more or less needles ; E is usually too small to require correction ; A is subject to deviation differing in amount and direction for fortunately rarely of any value, as it cannot be corrected. every azimuth of the ship’s head. This was first demon- The deviation observed when the ship inclines to either • strated by Commander Matthew Flinders by experiments side is due—(1) to hard iron acting vertically upwards or made in H.M.S. Investigator in 1800-3, and in 1810 led downwards; (2) to vertical soft iron immediately below that officer to introduce the practice of placing the ship’s the compass; (3) to vertical induction in horizontal soft head on each point of the compass, and noting the amount iron when inclined. To compensate (1) vertical magnets of deviation whether to the east or west of the magnetic are used ; (3) is partly corrected by the soft iron correctors north, a process which is in full exercise at the present of D ; (2) and the remaining part of (3) cannot be conday, and is called “swinging ship.” When speaking of veniently corrected for more than one geographical posiAlthough a compass may thus be the magnetic properties of iron it is usual to adopt the tion at a time. terms “ soft ” and “ hard.” Soft iron is iron which made practically correct for a given time and place, the
