Gunners callipers, East India Company, 1832 (c)

Inscribed with the bale mark of the East India Company and 'Crichton, London'. John Crichton was a London instrument maker based in Limehouse.

Gunners' callipers are scientific instruments for measuring the bore of cannon, the diameter of shot and clearance, or 'windage', between them. They are often engraved with tables of information for gunners. They were in use from 1700 (c) until muzzle-loading artillery became obsolete in the late 19th century. Made of two flat brass limbs with steel tips, the calibrations at the hinge give the ratio of diameter to weight of shot.