This instrument is very close to the design of Zubler's 'geometrical gunnery instrument'. It differs principally in having shorter side legs, so that it cannot be used as a pair of large dividers or calipers. The joint linking the side and central legs is also different: whereas Zubler shows all three limbs mounted together on the same axis, in this instrument the two side legs are held in place by blued-steel spring blades.
The central leg carries a double scale of polygons and a scale of degrees. As in Zubler's depiction there is a compass at its end, but in this case it is provided with a string-gnomon dial for about 48° latitude. There are also scales for Prague, Rome and Vienna inches.
Another instrument of this type, now in Florence, is signed 'I. B. F.', possibly an abbreviation for 'Iost Bürgi fecit' (Miniati pp. 32-33). In the quality of their construction, materials and engraving these two instruments are certainly compatible with other instruments by Bürgi (cf. catalogue no. 52).
Length: 356 mm
Inventory no. 47,493