An instrument used for determining the variation of the compass, in order to rectify the ship's course, &c. It consists of two circles, either laid upon or let into one another, and so fastened together in their centres that they represent two compasses, the one fixed, the other movable; each is divided into 32 points of the compass, and 360°, and numbered both ways from the north and the south, ending at the east and west in 90°. The fixed compass represents the horizon, in which the north and all the other points are liable to variation.