The instrument consists of an arched base into which an arc and plummet support are screwed.
The arch has a transverse groove running across its middle point, which may have been intended for sighting along the barrel of a gun.
The levelling arc is slightly more than 60? with a scale of equal parts running 24 to 0 to 24, divided to 4 and 1 and numbered by 4. When the instrument is placed longitudinally on a gun barrel, elevations are indicated by the pointer on the hinged sighting arm.
Unlike many gunner's sights and levels, in which there is both a plummet and a separate vertical sighting scale, these two elements have here been combined, presumably to avoid the inconvenience of the plummet obscuring the sight when the instrument is level.
The freely moving sighting arm is mounted at the top of the two supports, which are undecorated except for outline engraving. The slotted sighting arm has a sliding sight which moves against a scale from 1/2 to 21/2, divided to 1/2, 1/4 and eighths. The divisions correspond to Nuremberg inches.
The instrument is incomplete and a carrying ring would presumably once have been fitted into the hollow socket at the top.
Provenance: presented by Claude Fry (accession number 24-16a).
See J. Bennett and S. Johnston, The Geometry of War, 1500-1750 (Oxford, 1996), p. 36.