Signed: I. Rowley fecit
The semicircular scale is divided to half-degrees and numbered by 10 in both directions, i.e. 0-180 and 180-0. The set of jointed parallel rules relate the internal angle, external angle, and angle subtended at the centre for the walls of a regular polygonal fortification. This form of instrument was known in England as a 'parallelogram protractor'. Another instrument maker, Thomas Tuttell described it in his Description and Explanation of Mathematical Instruments (1701) and explained that the semicircular scale indicates 'the quantity of any inward or outward angle'.
Diameter: 174 mm
Orrery Collection (Christ Church)
Inventory no. 32,319