The marine sextant was developed in the late 1750s. It was used to make observations of the moon and sun. The angle is read off the curved scale at the bottom of the instrument and the scale on the arm, and allowed the navigator to estimate the latitude of the vessel (its position north or south of the Equator).

The sextant has a polished brass straight-bar pattern frame and limb, with a flat brass cross on the back for reinforcement. It also has a wooden handle with a brass-lined threaded hole for the second handle. The tangent screw and clamping screw are located on the back of the index arm. The sextant has four round shades, three red and one green, and two red horizon shades. Index-glass adjustment is made according to Gabriel Wright’s patent and on the horizon glass by a capped screw. Attached to the sextant is a magnifier on a 90mm swivelling arm, as well as a threaded telescope bracket in two parts, fitted for correcting collimation error. It has perpendicular adjustment by a rising-piece and a milled knob. The telescope is 143 mm in length with an inverted image, set focus, and two parallel cross wires. A second telescope is 90 mm in length with an erect image and set focus. The sextant is contained in a keystone mahogany box.

The instrument has an inlaid silver scale from -4°40 arcminutes to 141°40 arcminutes by 20 arcminutes, measuring to 122°. The sextant has a silver vernier measuring to 30 arcseconds, with zero at the right.

This sextant is signed on the limb 'Kings Patent. Gilbert & Wright London'. This refers to Gabriel Wright’s patent (no. 1229 of 1779) for an azimuth compass with an octant. The index glass is mounted on a platform on top of the index arm, and can be rotated round the pivot by screws for parallel adjustment. This patent adjustment is also found on Wright’s instruments preserved in the Science Museum London, inv. no. 1923.475, The Mariners’ Museum, Newport News, Virginia, inv. no. 1998