This instrument provides a link between more generalized tools of the surveyor and those specifically linked to fortification. A scale for laying out a polygonal structure on the ground or on paper must indicate, if not actual military use, at least an intention or acknowledgement of such a possibility.
Here the instrument is basically a simple theodolite (see catalogue no. 45), where a pivoted alidade moves over a circular degree scale. However, with the four fixed sights it also acts as a surveyor's cross or square, the reverse has a plumb-line, so that the instrument can be used as a level or for taking altitudes (a vertical orientation is permitted by the ball-and-socket joint), and the central compass for orientation is surrounded by the hour divisions of a sundial with a folding pin gnomon. In addition, an innermost scale, marked 'Polygon', sets out the angles made between the sides of three to twenty-sided regular polygons. There is a turned wooden case, lined with red velvet.
Diameter: 116 mm
Inventory no. 36,730