A brass framed sextant signed W.F. Cannon, 175 High St., Shadwell London E. The instrument has an 8.5 inch radius, a silvered arc with a framed steel vernier and magnifier, a telescope mount with three original telescopes, seven shades and mirrors.
Sextants were used to determine latitude through the determination of the angle between a known celestial object and the horizon. This could be the sun at solar noon, so the instrument has dark glass shades to permit the safe viewing of the sun's position. By aligning the image of the sun and the horizon by moving the vernier arm, the angle is found, then compared to a table for a given day to calculate latitude.