COMPASS. A name given to instru ments contrived to indicate the magnetic meridian, or the position of objects with respect to that meridian. According to the purposes to which the instrument is chiefly applied, it becomes the mariner's compass, the azimuth compass, the varia tion compass, each particular application requiring some peculiarity of construc tion ; but whatever modifications it may receive, the essential parts are the same in all eases. These are a magnetized bar of steel, called the needle, having fitted to it at its centre a cap which is supported on an upright pivot, niade sharp at the point in order to diminish the friction as much as possible, and allow the needle to turn with the slightest force. The mariner's compass has a circular card at tached to its needle, which turns with it ; and on the circumference of which are marked the degrees, and also the 32 points or rhomhs, likewise divided into half and quarter points. The pivot rises from the centre of the bottom of the cir cular box, called the compass box, which contains the needle and its card, and which is covered with a glass top to pre vent the needle from being disturbed by the agitation of the air. The compass box is suspended within a large box, by means of two concentric brass circles or gimbals, the outer one being fixed by ho rizontal pivots, both to the inner circle which carries the compass box, and also the outer box, the two sets of axes being at right angles to each other. By means of this arrangement the inner circle, with the compass box, needle, and card, al ways retain a horizontal position notwith standing the rolling of the ship.
The principal requisites of a compass are intensity of directive force, and sus ceptibility. The first of these is obtained by constructing the needle of the mate rial and form best suited to receive and retain the magnetic virtue. A number of experiments on this subject were made by Coulomb, and more recently by Cap tain Rater, an account of which is given in the Phil. Trans. for 1821. Captain
Rater found that the kind of steel capable of receiving the greatest magnetic force is shear steel ; and that the best form is that of a lozenge or rhomboid cut out in the middle, so as to diminish the extent of surface in proportion to the mass, it being found that the directive force of the needle, when magnetized to satura tion, depends not on the extent of surface, but on the mass. Beyond a certain limit (about five inches) no addi tional power is gained by increasing the length of the needle ; and needles ex ceeding a very moderate length are apt to have several consecutive poles, the ef fect of which is to produce a great dimi nution of directive force. On this no count short needles, made very hard, are to be preferred.
The azimuth compass, being intended to show the bearing of objects in respect of the magnetic meridian, has its circle divided merely into degrees, instead of the rhombs used in navigation, and is provided with sights to allow the angles to be taken more accurately.
The variation compass, is designed to exhibit the diurnal changes in the devia tion of the magnetic from the true me ridian ,• and the needle is generally made of much greater length than the mariner's compass, in order to render minute va riations more sensible.
Mr. Dent, of England, in 1845, made an improvement in the compass, which consists in placing the it i.gnetic needles and the card on an axis, instead of the usual mode of suspension, the point be ing higher than the centre of gravity, and subject when on shipboard to the law of pendulous bodies. Mr. Dent has also so improved the azimuth compass, that by turning an azimuth 180°, it effects the correction fbr collimation ; and by in verting the card, it being engraved on both sides, affords the means of determining the error of the zero on the arc], not co inciding with the magnetic meridian.