Rulers like this one are used to plot course lines and bearings on nautical charts. They can be rolled across the chart to the compass rose without changing its orientation.
Field (1796-1871) improved the traditional design of parallel ruler by adding a protractor style scale to one edge of the rule and compass points to the opposing edge.

Catalog: NM. 3-17
Date: ca. 1900
WL: 6×38 cm (2,5×12 in)
Signed: Capt. Field improved parallel
Capt O.M. Watts Ld. London
British made
Origin: England
Condition: in good and working condition, wear consistent with age and use