Gunner's callipers, early 18th century.
© Science Museum / Science & Society Picture Library
These callipers can be used to measure the diameter of cannon balls, or with crosed arms as shown here, to measure the bore of a cannon. The arms are inscribed with tables containing information relating to artillery. The instrument was made by Thomas Wright (d 1767), who styled himself mathematical instrument maker to George, Prince of Wales, in 1718.