Graduations-Semi-circle graduated 0-180, 180-0, 0-60-0, 0-60-0, and inscribed "VMBRA VERSA" VMBRA RECTA", "VMBRA VERSA". The alidade 8.3" (21.2cm) long, has a fiducial edge at each end. A suspension ring is attached to upper part of semi-circle and there is a straight edge 9.45" (24cm) long at lower part. A ball and socket head and a square plate 3.55" x 3.55" (9 x 9cm) are attached to back of instrument.
Sights-Two sight vanes 1.55" (4cm) long mounted on straight edge, one sight with a vertical slit with a rectangular aperture above (complete with brass sighting strip), the other with a rectangular aperture (also with sighting strip) with a vertical slit above. Two sight vanes 1.5" (3.8cm) long mounted on alidade, similar in description to those on straight edge.
Compass-Dia. 2.75" (7cm) mounted at centre of instrument. Fitted with a narrow blued steel needle in form of an arrow 1.95" (5cm) long, with a brass pivot cap at centre. Graduated to 32 points, quadrantally in degrees and to 360 degs.
This instrument was an early form of theodolite with which surveyors could measure angles between two specific points. This was usually done horizontally, although this example has a ring which allows it to be suspended to also measure vertical angles. The compass enabled the instrument to be positioned accurately.