Combined Sector, Folding Square and Diagonal Scale, by Thomas Heath, London, c. 1720-50

Sector on one side, diagonal scale on the reverse; 5 inch radius; with bevelled edges carrying scales of equal parts. This was presumably once part of a 4½ inch pocket set of instruments (a standard size of the period). The multiplicity of functions served by the instrument suggests that it might have been accompanied in a case by perhaps no more than a pair of compasses.