Compass, a magnetized needle, balanced upon its centre so as to swing freely, used to indicate the magnetic meridian, and, by means of a graduated circle connected with it, the azimuths or bearings of objects from this meridian. The Chinese appear to have been acquainted with the property of polarity in the loadstone, and in iron or steel magnetized by it, and to have been the first to apply this. Some affirm that they employed only the loadstone (magnetic iron ore), floating it upon a piece of cork; and that the magnetized needle is the invention of Flavio Gioja of Amalfi, who lived in the early part of the 14th century. Dr. Gilbert, in his De Magnete, etc. (London, 1600), states that the compass was brought to Italy from China by Marco Polo about 1295. But there is evidence of its having been used in France about the year 1150, in Syria about the same period, and in Norway previous to 1266. Several forms of the compass are in use. The mariner's, made for service at sea, and especially for indicating the direction in which the ship's head points, consists of a needle attached to the under side of a circular card or disk, upon the top of which the cardinal points and their subdivisions are marked. A fleur de lis is on the N. pole of the needle, and the letter S. on the opposite pole.

E. and W. are placed, the one to be toward the east, and the other toward the west, when the card, swinging with the needle, comes to rest, dividing the circle into quadrants. A diameter is drawn, bisecting each of these, fixing the N. E., S. W., N. W.,. and S. E. points upon the circumference; and the arcs thus obtained are again bisected by new diameters, again doubling the number of points; and the process is repeated upon the smaller arcs, obtaining in this way 32 points or divisions of the circle, each representing an arc of 11° 15'. The points are designated as follows for one quadrant, and on the same plan for the rest: N, Nb (by) E, NNE, NEbN, NE, NEbE, ENE, EbN, E. Lesser divisions sailors indicate by half and quarter points, thus: E. N. E. 1/2 E., N. E. 1/4 K, E. 3/4 N"., etc. The degrees are also usually numbered around the margin of the circular card. An agate, or better a garnet cap, is set in. the middle of the needle to receive the sharp pivot standing in the middle of the compass box, upon which the needle and card are balanced. This box is of copper or brass, of cylindrical or hemispherical shape, and covered with a glass plate to exclude currents of air and dust.

It is supported in a ring by two pivots projecting from opposite sides of the box, and this ring is swung by two other pivots placed so that the line connecting them is at right angles to that connecting the other two. This contrivance, called "gimbals," is designed to keep the central pivot always vertical in the movements of the ship, the box being made heavy at the bottom, so that its centre of gravity is considerably below the points of suspension, in which it swings freely. The pivots of the outer ring are fixed to a frame or to the inside of a square wooden box, in which the instrument is placed. Instead of using gimbals, a cap with a pivot standing in the ,top of it is sometimes placed upon the stationary pivot, and the needle is balanced upon the top of the upper one. On board ship the compass is set in a receptacle called the binnacle, and the direction in which the vessel heads is indicated by a distinct vertical mark on the inside of the inner box, close to which the points upon the card pass as this swings round. - The compass used in land surveying is made with the graduated circle fixed to the plate of the instrument, the needle pointing the degrees at each of its extremities.

Two sights are erected opposite each other on the plate which supports the box, on the line of 0°180°, and the needle when at rest points to the degree representing the azimuth of this line. With an idea of facilitating the reading of the bearing of objects seen through the sights of the instrument, the letters E. and W. in land compasses are placed opposite the positions they occupy on the card of the mariner's compass. Reading always from the N. pole of the needle, and supposing the line of the sights to be directed toward S. 45° W., this N. end will be found half way between the letters S. and W. A very convenient form of the azimuth compass is a modification of that invented by Capt. Kater, and sometimes called the prismatic compass. The needle is suspended upon a pivot in a shallow cylindrical box, and supports by its extremities a silver ring graduated to 360°, with the half degrees and sometimes the quarters also marked by short lines. The N. pole of the needle is on the zero point, and the S. pole on 180°. A sight frame with a fine hair in an elongated vertical opening is attached by hinge or otherwise on one side of the box, and opposite this is a low sight piece, so arranged with a short slit in the top and an eyehole directly under it, in which is a small transparent prism, that the eye can at the same time observe the range of the sights upon any distant object, and read through the prism the number of the degree directly under it: thus if the bearing be N. 15° E., the number seen is 185. This instrument is of great service for rapid observations; it is carried in the pocket, is held in the hand when an observation is made, and maybe read to 15' by one accustomed to its use. - The force with which compass needles tend to range in the magnetic meridian was found by Coulomb and Kater to be influenced in those of the same form, not by the amount of surface, but by their mass, when fully charged with magnetism.

Shear steel was found to be the best material, and the form of the needle an elongated lozenge, the. middle portion cut out, and a bar of brass inserted across the centre to receive the cap. No advantage is gained in making them more than five inches long; on the contrary, several poles are apt to be produced, the effect of which is to lessen the force of the single polarity sought for. - The compass is liable to err in its indications from causes, some of a local and others of a general nature. The former are mostly beyond the skill of man to control; and, often acting when no suspicion of their existence is entertained, the compass cannot be regarded as an accurate instrument in running lines. It has, in fact, gradually been giving place to other methods of determining these. The effect of the declination of the needle, or its variation from the true meridian, is corrected by allowing for the amount of this variation as established for the place and time, or as determined by observations made for the purpose. On certain lines upon the earth's surface, called lines of no variation, the needle points toward the pole.

Such a line at the present time passes near Wilmington, N. C, Charlotteville, Va., and Pittsburgh, Pa. On the eastern side of this line the variation of the needle is toward the west, increasing in amount with the distance from it. At New York the variation is 6° W. and at Portland, Me., it is 13° W. On the other side of the lino of no variation, the declination is toward the east, being 5° E. at Key West, and reaching on the Pacific coast 15° or 20° E., or almost N. N. E. This variation undergoes a progressive change in amount, and, after long periods, changes in direction, vibrating, in fact, between certain limits. In the eastern states the north pole of the needle is moving westward at the rate of about 1° in 12 years. In London, in 1576, the variation was easterly 11° 15'; in 1657-62 it was reduced to nothing, and then slowly advanced to its maximum in a westerly direction, which in 1815 was 24° 27' 18". Since that, time it has been slowly decreasing. It is thus perceived that surveys made by the compass should always be referred to the true meridian, or their date be given, that such reference may at any time afterward be made; but the latter method is not altogether trustworthy.

The subject of these movements for this continent is treated in the reports of the proceedings of the American association for the advancement of science, at their ninth and tenth meetings. - The variation of the compass in ships from local causes is a matter of the most serious consequence, and baffles the skill of man to control its effects. It is not a constant determinable error, but varies with the position of the vessel; it is generally greatest when she heads E. or W., and least when she lies on the magnetic meridian. It is different also as the ship lies on even keel or careens over on one side. The liability of error increases with the increased use of iron in the construction of ships, and those built of it are found to act themselves as magnets, especially if they stood upon the stocks with the head toward the north. In this case, sailing for some time in a contrary direction tends to change their polarity and disturb the compasses. The plates are made magnetic by the blows they receive as the ship is constructed. The attention of scientific men has been directed for years past to the devising of some protection against this evil. In iron ehips compasses are stationed in different parts, and comparative observations are constantly made of their indications.

The most confidence is placed in those which are furthest from the hull, as at the mast head. In the British navy it has been the practice for many years to occasionally swing around each ship, and note the indications of the compass as she heads in different directions, and thus form a table of errors to be applied to correct the compass when she is afterward sailing on these courses. The board of trade recommended the adoption of this practice for merchant ships. Professor Airy, astronomer royal, objected to it, and recommends instead the use of magnets placed near the compass, and so arranged as to neutralize the influence of all other local attractions. He also advises frequent examination of the compasses, and testing them as often as practicable by azimuth observations of a star or other objects. In some ships a neutral point has been found in which the local attractions were all balanced, and so continued to be, rendering this a suitable spot for the compass. The discovery of a sure method of obtaining a neutral point of this character is an object of the highest consequence; but, however secured, there are so many causes to influence the condition of the needle, some slow and some sudden in their action, such as electrical currents induced by atmospheric agencies, the heavy shocks of the waves upon the ship, etc, that frequent observations and constant care are essential to make the compass a safe guide.

Sir John Ross found that the needle was attracted full 5° by the rays of the moon concentrated upon it. The British admiralty have caused experiments to be made, and have invited competition, in the construction of the compass; and in consequence, from being a common, it has become an artistic instrument in that country, great improvements having been made in it.

Mariner's Compass.