COMPASS, MARINER'S, is the name given to the instrument by which sailors are enabled to steer their course on the ocean out of sight of land, and when neither sun nor stars arc visible. The ancients, to whom it was unknown, could seldom venture to lose sight of the coast. The directive power of the magnet seems to have been unknown in Europe till late in the 12th century. It appears, however, on very good authority, that it was known In China, and throughout the cast generally, at a very remote period. The Chinese annals indeed assign its discovery to the year 2634 B.c., when, they say, an instrument for indicating the s. was constructed by the emperor Hou-ang-ti. At first, they would appear to have used it exclusively for guidance in traveling by land. The earliest date at which we hear of their using it at sea is somewhere about 300 A.D. According to one account, a knowledge of the C. was brought to Europe by Marco Polo, on his return from his travels in Cathay. It was long contended that the C., as a nautical instrument, was first invented by Flavio Gioja, a native of Amalfi, about the year 1362; and that that part of the kingdom of Naples where he was born has a compass for its arms. For this there is no authority whatever, as the C. was well known as a nautical instrument before his time. It is probable, however, that Gioja may have made some improvement in the instrument or in the mode of suspending it. See British Annual for 1837.
The essential part of the C. is the magnetized needle, balanced freely upon a fine point; such a needle has the property of arranging itself in the meridian, one end always pointing to the n., and the other to the s. (see MArnawrism); yet not exactly, but with a deflection or declination (q.v.), which varies from time to time in magnitude, and may be towards the w. or the east. The form of the needle is various, some being long fiat prisms, others lozenge-shaped. The center of the needle is pierced, and a piece of agate generally inserted, with a conical hole sunk in it, to receive the fine point of the steel pin, so that the free motion may not be hindered by friction. The construction of the rest of the instrument depends upon the purpose it is to serve. For a mariner's C., the needle is fixed to a circular card, on which there is a star of 32 rays marking the 32 points of the heavens. The n. point of the card is immediately over the n. end of the needle, and the card moves with the needle. The cardinal points are marked with the letters n., s., C., w.; and the intermediate points other letters. The
points are often subdivided into quarters, which are thus marked: nje. (read "n. one fourth c."); n.ie.; D. by e./e., etc. A point of the C. being of the circle, is equal to 11° 15'. The C. thus formed is inclosed in a cylindrical brass box; and in order that the C. may remain horizontal in all positions of ,the ship, the box is suspended by gimbals (q.v.). The whole is then placed in the binnacle (q.v.), in sight of the helmsman. Inside the box, in the direction of the ship's bow, is a vertical black line called the lubber-line, and the steersman must keep the point of the card which marks the prescribed course always in contact with the black line. Compasses differing in some respects from the mariner's C. are also used iu surveying and land-measuring; and for the miner they are perhaps as necessary as for the sailor.
The great difficulty connected with the use of the mariner's C. arises from the dis turbing influence of the magnetism of the ship. The difficulty is particularly felt in iron vessels, where the deviation of the needle is frequently so considerable as to render the compass almost useless. Various means of obviating this have been suggested; one of these is to place bars of soft iron or magnets in the immediate neighborhood of the binnacle, which being so placed as to cause a contrary disturbance to that of the iron of the ship, leave the needle comparatively free. This is found to answer well in iron ships plying between British and continental or North American ports; but where, as in the Australian passage, they change considerably their latitude, such an arrangement is found to be worse than useless, as the magnetism of the vessel changing with the magnetic latitude, causes an ever-varying deviation of the needle. It has likewise been suggested to place a compass as a standard at the mast-head, where it would be com paratively free from the attraction of the vessel, by which the ship's course might be shaped, the ordinary C. being used merely to give immediate direction to the steers man. In the royal navy, this error is to a large extent obviated in the following way. A C. is placed so high above the deck as to clear the bulwarks, and allow the bearings of a distant object on shore or a heavenly body to be taken while the ship's head makes a complete circuit. In this way, the deviation caused by the iron of the ship in all dif ferent positions may be ascertained, and afterwards taken into account.