The mariner’s compass is an artificial magnet fitted in a proper
box, and consists of three parts--the box, the card or fly, and the
needle. The box is suspended in a square wooden case, by means of two
concentric brass circles called gimbals, so fixed by brazen axes to the
two boxes, that the inner one, or compass-box, retains a horizontal
position in all motions of the ship. The card is a circular piece of
paper which is fastened upon the needle, and moves with it. The outer
edge of the card is divided into thirty-two points, called points of
the compass. The needle is a slender bar of hardened steel, having a
hollow agate cup in the center, which moves upon the point of a pivot
made of brass.