The instrument consists of a graduated quadrant with a plummet and an upright hollow cylinder, both fixed to a slightly curved rectangular base. The cylinder has no obvious fixings and the quadrant is crudely riveted to it.
The rectangular base is only decorated with a simple line pattern with roundels at the corners. The quadrant is pierced and highly decorated with a foliate design.
Mounted on top of the curved base and flanked by two adorsed dolphins is a hollow cylinder pierced on both sides with a full-length vertical slot. One of the slots is graduated 1 to 13 on each side from the bottom up, the other is plain. Inside this outer cylinder is a second cylinder pierced through both sides with a series of holes arranged in a two-start helix. On one side of the outer cylinder, over the graduated slot is a plumb bob, itself cut with a slot to reveal the sighting holes beneath and narrow enough not to obscure the graduated scales at the sides. At the top of the outer cylinder is a cap formerly retained by two screws which are now missing. This cap is surmounted by a serrated finial which turns the inner cylinder and which also has a pointer which registers against two marked positions on the fixed cap.
The quadrant is graduated 12 to 1 and 1 to 12 and divided to 1/5. A corresponding inner scale is graduated 6 to 1 and 1 to 6 with alternating plain and shaded rectangulars.
The hinged two-part outer case is made of wood covered with stamped black leather. It is has two catches for closing.
The instrument was donated in 1894 by Max Rosenheim and is described in F. A. B. Ward, A Catalogue of European Scientific Instruments in the Department of Medieval and Later Antiquities of the British Museum (London, 1981), p. 106, no. 313 (with plate XLVIII).