Another instrument which links general surveying with fortification is this graphometer with a scale for polygons. Both the diametric rule and the pivoted alidade have linear scales and pairs of complex sights, each of which moves on a short vertical scale (one sight is missing). The underneath of the instrument is flat and the alidade moves on an open ring rather than a central pivot, so that the centre of the semicircle is always exposed; the instrument can thus be used as a protractor.
The semicircular scale is divided to degrees and the subdivision of each degree into minutes is by a steel index arm, which is given an epicyclic motion over a circular scale divided 0-60 four times and rotates as the alidade is moved around the centre of the semicircle. The semicircle has a second set of divisions for laying out, in plan or in the field, the angles of regular polygons of between three and twenty-four sides.
Diameter: 300 mm
Inventory no. 47,382