A computing instrument, comprising a brass quadrant of 230mm radius, 235mm along its other edges. It has three feet and scales along the curved outer limb: an outer scale (D) calibrated 0 to 90 clockwise; an inner scale (B) 90 to 0 clockwise. Each scale is divided to 1/5 degree, also graduated in compass points 8 to 0 clockwise and 0 to 8 clockwise. An inner curved limb has two degree scales marked 0 to 90 (b), and 0 to 90 (a), both to ½ degree.

The first radial limb is marked 'M P' and has 4 scales :
Outer : – 80
Next: - 60 -70
Next: -20-30-40-50-60
Next: short scale 1 10 –lat 0

The other radial arm has a slider to which is attached a sliding arm calibrated 0 to 60 and 0 to 130 and labelled 'R'. This intercepts the two circumferential scales. This slider is inscribed (in two parts) 'Plain Sailing' then 'Middle Latitude', 'Parallel' and 'Mercator’s Sailing'. This radial arm carries scales marked 0 to 130 and 0 to 260, against which a cursor, which is part of the swivelling arm, can be read.

Pivotting label with 2 rivets missing
Pivot from apex missing
Claculating quadrant for solving problems concerning plain sailing, middle latitude, parallel & Mercator's sailing
Plain on reverse