A graphometer consists of a graduated semicircle with a pair of sight vanes
at either end, and a movable alidade with another pair of sights at either end.
The form was introduced in Philippe Danfrie, Déclaration de l’usage du
graphomètre (Paris, 1597), and was always popular in France. Many
graphometers have an inset magnetic compass. Most graphometers are made of
brass, but some American ones are made of wood. Some 19th-century graphometers
had telescopes rather than open sights. Graphometers were also known as semi
circles or semicircumferentors.
Ref: J. A. Bennett, The Divided Circle (Oxford, 1987), pp. 49–50.