COMPASS 91 COMPASS or fly, and the needle — ^the latter being the really essential part, and consisting of a small magnet so suspended that it may be able to move freely in a horizon- tal direction. The box, which contains the card and needle, is, in the case of the common mariner's compass, a cir- cular brass receptacle hung within a wooden one by two concentric rings called gimbals, so fixed by the cross cen- ters to the box that the inner one, or compass-box, shall retain a horizontal position in all motions of the ship. The circular card is divided into 32 equal parts by lines drawn from the center bar of magnetized steel. It is fixed on the under side of the card, and in the center is placed a conical socket, which is poised on an upright pointed pin fixed in the bottom of the box; so that the card, hanging on the pin, turns freely round its center, and one of the points, by the property of the needle, will always be directed toward the North Pole. The needle, however, is liable to a certain deviation owing to the magnetism of the ship itself, and this is especially strong in iron ships. To obviate this defect Sir William Thomson invented a compass, having a number of needles arranged in COMPASS to the circumference, called points or rhumbs; the intei*vals between the points are also divided into halves and quarters, and the whole circumference into equal parts or degrees, 360 of which complete the circle ; and consequently, the distance or angle comprehended between any two rhumbs is equal to 11%°. The four principal are called cardinal points: viz., North, South, East and West. The names of the rest are com- pounded of these. The needle is a small a particular manner instead of one. In this compass quadrantal erroi's are cor- rected by means of two iron globes fixed on opposite sides of the binnacle; while the various components of the ship's magnetic force are neutralized by a series of bar-magnets so arranged as to act as correctors. In the compass used by land-surveyors and others the needle is not fixed to the card, but plays alone, the card being drawn on the bottom of the box.
