Circumferentor

The Circumferentor was an early theodolite. It was a large compass with a swivelling dial. A metal gunsight groove was used for sighting on distant objects or on progress lines. The dial could be adjusted to record a bearing from magnetic north for the progress line  and then swivelled to bear on the object.  Subtraction determined the angle of the progress line  to the sighted object. The instrument was mounted at eye-level on a pole; a ball-and-socket joint eased the swivel. It was a very inaccurate instrument.