Sextant used by General Sir Robert Biddulph, Royal Artillery, 1860

Brass sextant made by Troughton and Simms, London.

Cylindrical brass casing, with lid that unscrews. Base of sextant has a rectangular slot, which opens to admit light. On top a screw rotates mirror inside; vernier pointer connected with mirror which moves around a scale marked from 0 to 140 degrees, right to left, in 70 degrees of actual arc. Pivoting with the pointer is a brass arm with a magnifying glass, to read the vernier. From the right hand side a small telescope can be extended for focusing; the eyepiece is protected by a small brass cover. Part of the left side is removed to admit light to a half silvered glass plate, whose inclination to the plane of the instrument can be varied by a key which is screwed into the top of the casing.

Military engineers used this instrument to work out the angle between the horizon and either the sun, moon, or a star. The angle could then be used, alongside the time of day it was taken, to determine latitude or longitude.

General Sir Robert Biddulph (1835-1918) was commissioned into the Royal Artillery in 1853. He served in the Crimean War (1854-1856) and the Indian Mutiny (1857-1859). He became Assistant Adjutant-General in 1871 and later served as Governor of Gibraltar.