Circumferentor

A circumferentor, or surveyor's compass, is an instrument used in surveying to measure horizontal angles. It was superseded by the theodolite in the early 19th century.

A circumferentor consists of a circular brass box containing a magnetic needle, which moves freely over a brass circle, or compass divided into 360 degrees. The needle is protected by a glass covering. A pair of sights is located on the North-South axis of the compass. Circumferentors were typically mounted on tripods and rotated on ball-and-socket joints.