A nine inch bras sector, by George Adams; engraved 'Improv'd and made by Geo. Adams, Mathm. Inst. Maker to His Royal Highnes, George Prince of Wales, London'. Sectors were used in calculations involving proportion. They are composed of two flat arms hinged together that can be adjusted to form any angle. Each arm is engraved with logarithmic and trigonometrical lines, which radiate from the centre of the hinge.
© Science Museum / Science & Society Picture Library