The instrument is of turned wood and would originally have had a levelling device for insertion in the central hole (cf. Epact 91515).
The upper face is inlaid with bone and has an outer circular scale  to 24, divided to 1, 1/2, 1/4 and 1/8, and numbered by 1. The central circle has foliage decoration and two inset compasses 30 mm in diameter aligned with the 12 to 24 meridian line. Both compass bowls have the inscription 'M A' in red within green scrolls. Each compass is also flanked by the initials 'G G'.
Formerly in the Lewis Evans Collection (I.154).