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

A brass square with a rotating circular disc carrying divided lines for the five classical orders of architecture. The instrument is not signed but forms a pair with inv. 18805.1.5. Both sides are labelled o p q r in the corners and each side carries a circle divided into 4 quadrants, divided in the same way into 12 unequal divisions (0-12-0-12-0) and subdivided to ½s, with the two zero points on each side marked n. Within these divided circles rotates a disc carrying divided lines for the classical 5 orders. On one side are pairs of divided lines for the Tuscan order (A-F), Dorick (G-O) and Ionick (P-X). On the other are Corinthian (a-h), Composite (i-q) and Scales (r-s). The rotating disc has two finger holes to permit its rotation and at the corners marked p and q are slits which would act as springs when the device was inserted in the slot of a special drawing board.