COMPASS, Mariner's, an instrument to ascertain directions at sea by means of the at traction of the earth for a movable magnet or a set of magnets. Similar instruments consist ing of a magnetized needle resting on a pivot are used by the soldier or woodsman to de termine directions on land. In the mariner's compass a thin circular sheet of mica is sup ported so as to turn with great freedom in a horizontal plane about its centre. This is called the compass-card. The bearing usually consists of a small plate of agate let into the card, and has a conical hole at the centre, and this rests on a fine needle-point of hard steel. This ar rangement gives very little friction. To the under surface of the compass-card several parallel magnets are attached. Many experi ments have been made to find the best arrange ment for the magnets, but there is probably still something to be learned on this question, though the scheme patented by Lord Kelvin, in which the needles are suspended from the card in such a way that the moment of inertia of the card about every diameter shall be the same, is the most generally used. The compass-card is marked with a star of 32 rays, which are called the thumbs, or the points of the compass. A line joining two of these points diametrically opposite is or ought to be exactly parallel with the magnetic axis of the arrangement below; and at the extremities of these points are marked N. (north) and s. (south). E. (east) and w. (west) are marked at the extremities of another diametral line at right angles to the first. The other points have also names; and to repeat these from memory in order, beginning at north and going round the whole circle, is what sailors call °boxing the compass." Most compasses are also graduated in degrees, either separately in each quadrant from north and south, or from 0° to 360° from north in a clockwise direction.
The card is supported, as we have said, on a steel point, which is attached either to a hemi spherical bowl of thick copper, or to a thick copper ring when the compass is to be used for night-sailing in which case a lamp placed be low the ring shines up through the mica card and makes the markings on it visible. This thick copper bowl or ring is called the compass box. Its use is to damp the vibrations of the needle; for it is found that the presence of a large mass of copper properly placed damps the vibrations of a freely suspended magnet rapidly, while it does not at all prevent it from coming to rest in the proper position. In certain forms of compass, part of the weight of the card is borne by a non-freezing liquid, which also serves to damp its oscillations. The compass
box is suspended on gimbals, which are two concentric copper rings. The larger turns on a horizontal axis, whose extremities rest on the inside of the case that contains the compass. The smaller ring turns on a horizontal axis at right angles to the former, and resting on the outer ring. The compass-box is attached to the inner ring, and its weight tends to keep both horizontal. Thus supported, the compass box and card always remains horizontal how ever the ship rolls or pitches. The chief diffi culty in making use of the compass arises from the magnetism of the ship itself. In iron ships particularly, the magnetism of the ship greatly interferes with its indications. The effect of the ship's magnetism is determined by swing ing the ship, and determining a correction to be applied for every position. This, however, is liable to some uncertainty, for it is found that rough weather and other circumstances alter the magnetism of the ship, and therefore its effect on the compass. It is usual to place a compass at the mast-head of large ships, and from time to time to compare the deck compass with it. Furthermore, the inherent magnetism of the ship is compensated for to a great degree by an extremely complicated arrangement of bars of soft iron and magnets placed in the binnacle.
The origin of this instrument cannot be traced with anything like certainty. It has been supposed that the Chinese knew something about the polaric property of loadstone more than 2,000 years before the Christian era. In the 12th century the compass became known in Europe. Whether it was learned from the Orient is not certain. The Italians ascribe the invention of the compass to Flavio Gioja, a native of Amalfi, giving the dates 1300-20, but it has been shown that he can be credited only with some important improvement of the in strument. The discovery of the variation of the needle has been generally attributed to Columbus, but is now supposed to have been known much earlier. See GYROSCOPIC COMPASS; Bowditch, 'The American Practical Navigator> (Washington, new edi tions from. time to time) • Cornwell, 'Compass Disturbances in Iron Ships' (1887) ; Diehl, 'Deviations of the Compass' (Washington 1902) ; Capt. F. J. Evans and Archibald Smith, 'The Admiralty Manual for the Deviations of the Compass' (new editions from time to time) ; Lyons, 'Treatise on Electromagnetic Phenomena and the Compass and its Deviations Aboard Ship' (New York •901-03) ; Smith, 'Practical Compass Adjustment ( Seattle 1903).