Consists of sivered brass bearing plate graduated to 1/4 points, and with two sets of graduations 0-180 deg from both N&S. The plate can be clamped either to the frame by a central knob or to the sight vane by a screw below. The centre knob also carries a "Mag" pointer. The sight vanes are connected by an arch which carries a thread. A brass conical gimbal mounting and lead balancing weight are attached to a spindle at centre of underside of plate. In wooden box 9.5 x 8.25 x 15.5" (24.2 x 21.0 x 39.4cm) containing a brass mounting plate.