COMPASS 187 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. E., N. E. 1ST., E. | N., &c. The degrees are also usually numbered around the margin of the cir- cular 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 com- pass 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 contri- vance, called "gimbals," is designed to keep the central pivot always vertical in the move- ments 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 out- er 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 ver- tical 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 sur- veying 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 po- sitions they occupy on the card of the mari- ner'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 cylindri- cal box, and supports by its extremities a sil- ver ring graduated to 360, with the half de- grees 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 other- wise 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 mag- netism. Shear steel was found to be the best material, and the form of the needle an elon- gated 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 pro- duced, 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 suspi- cion of their existence is entertained, the com- pass cannot be regarded as an accurate instru- ment in running lines. It has, in fact, gradual- ly been giving place to other methods of deter- mining these. The effect of the declination of the needle, or its variation from the true me- ridian, 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 line 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, be- tween certain limits. In the eastern states the north pole of the needle is moving west- ward 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'
