This rather unusual folding plane table alidade with a magnetic compass consists of two hinged arms with an integrated magnetic compass.
Both arms have a continuous linear scale 0 to 200, numbered by 10 and divided to 5 by alternate shading.
A sight is fixed by a screw to the end of each arm and there are additional holes further towards the centre where another pair of sights may have been fixed.
Both arms have additional pierced and shaped decorative attachments.
As part of the hinge a circular plate fixed to one arm is engraved with a concentric degree scale 0 to 90, 0 to 90, 0 to 90, 0 to 90, numbered by 10 and divided to 5 and 1. The four starting points are marked 'A', 'B', 'C' and 'D'. The silvered compass box with an index is mounted in a rotatable holder in the centre of this scale. It is marked in Latin for the four cardinal points and has an indication for the magnetic deviation 9? E of N. The compass has a detachable lid.
The signature of the maker 'Erasm<us> habermel Pragae fecit' is inscribed on the underside of the arms close to the compass.
Both sides of the instrument are decorated with stylized stars and flowers as well as scrollwork and strapwork.
The instrument was presented by A.W. Franks in 1871 and is described in W. Eckhardt, "Erasmus und Josua Habermel: Kunstgeschichtliche Anmerkungen zu den Werken der beiden Instrumentenmacher", Jahrbuch der Hamburger Kunstsammlungen 22 (1977), p. 55, no. 119; F. A. B. Ward, A Catalogue of European Scientific Instruments in the Department of Medieval and Later Antiquities of the British Museum (London, 1981), p. 100 ff., no. 290.