(Lat. gemellus, twin), the two brass rings or hoops within which a ship's compass is suspended in its box, in order to counteract the effect of the ship's motion and keep the card horizontal. These rings move one within the other, each perpendicularly to its plane, about two axes placed at right angles to each other. By this contrivance the compass, having a free motion in two directions at right angles to each other, maintains a vertical position, and consequently the card is horizontal. Gimbals are also applied to other instruments, such as the portable or mountain barometer, &c.