Architectural Protractor to the Design of William Halfpenny, by Thomas Heath, London, c. 1728

Brass rectangle with a rotating circular segment for drawing the heights of elements of the five classical orders of architecture. There is a semicircular scale made up of two symmetrically divided quadrants (12-0-12). The divisions are unequal, with each unit subdivided to ½, ¼ and eighth. The zero point is marked m. On the semicircular base of the plate are 5 lines numbered 1-5 at each end. Each line is divided at two points on its length, numbered from 1 to 10.The circular segment is labelled y at its centre and x and z on its two straight edges. There are two sets of scales separated by the decorated inscription of authorship and maker. On the x side are scales for the bases and pedestals of each of the 5 orders, the lines all divided for appropriate dimensions. On the z side there are similar scales for the impost, capital, architrave, frieze and cornice of each of the orders. There is also a scale of 10 equal minutes.

The corners of the rectangle are identified by letters s t u w, and there are roughly-cut slits at s and w which would act as springs when the device was inserted in the slot of a special drawing board.

The reverse is blank except for 9 screws to hold the two layers of the rectangle together, each of which is individually identified by dots, and a screw for the centre of the rotating segment.