The sextant has a semi-circular brass protractor and straight edge. There is no tangent or clamping screw. The base is a straight edge and is marked from 0 to 1760 and carries a diagonal scale to 1 yard. The sextant has no shades. Index- and horizon-glass adjustment is made by screws. The index glass is mounted on an arm that moves along the protractor and carries the vernier. The horizon glass and the sight vane are mounted at either end of a slotted index arm. The slot rides over a pin that is fixed to the arm of the index glass. The sextant is contained in a fitted leather case marked in ink, ‘G Talbot’.

The instrument has a brass scale from 0° to 130° by 30 arcminutes, measuring to 121°. The sextant has a brass vernier measuring to 1 arcminute, with zero at the right.

Sir Howard Douglas (1776-1861) designed this instrument (patent no. 3461 of 1811) for land and marine surveying, whereby the measured angle may be protracted in actual magnitude without reading off. Sir Howard, son of the equally inventive Admiral Sir Charles Douglas, was an army officer who studied navigation shortly before he designed this instrument, for which he was elected Fellow of the Royal Society in 1816.