Sector, by Henricus Sneewins, Leiden, Mid-17th Century

Brass sector with decorative hinge and shaped ends to the legs; the retaining pin between the legs has been sheared off. The signature side has sectoral scales for fortification, metals, division of a circle, chords, cubes, the division of a straight line and mean and extreme proportion; it also carries single scales for iron and lead. The reverse has scales for the comparison of polygons and polyhedra, "Sub Ang Pol" (3 to 30), tetragonal, arithmetical, geoemtrical and for the inscription of polyhedra in a sphere; a single scale gives tangents to 65.