Graphometer

The graphometer, semicircle or semicircumferentor is a surveying instrument used for angle measurements.

It consists of a semicircular limb divided into 180 degrees and sometimes subdivided into minutes.

The device is mounted on a staff via a ball and socket joint.

[1] The form was introduced in Philippe Danfrie's Déclaration de l’usage du graphomètre (Paris, 1597)[2][3] and the term graphometer was popular with French geodesists.

[4] Le Nôtre's La theorie et la pratique du jardinage ('The theory and practice of gardening'), published in 1709, described the use of the graphometer in transferring geometric shapes from garden plans onto landscapes at a large scale.

Butterfield compass graphometer
A German graphometer in Göttingen, Stadtmuseum. The instrument is on its side. At the back, the socket for a Jacob's staff can be seen.
19th century graphometer
Figure 1: Angle EKG