Two-ft boxwood slide rule, for navigators, late 18th century

A 2 foot boxwood slide rule for navigators, c.1800. 'Sliding Gunters' , or navigator's rules with a slide are comparatively rare, as most navigators preferred to stick with the ordinary 'Gunter' scale based on the combination of trigonometric, logarithmic and surveying lines suggested by Edmund Gunter in the 1630s.