Jacob de Steur, who worked in Leyden in the second half of the 17th century, is known to have made several instrument of this sort. This one is particularly elegant, with images of dolphins at either side of the four fixed sights, and four dolphins on the alidade. The outer circle is graduated to 15 minutes. One half of the inner circle has a grad or gradient scale (graduated 0–100–0–100–0 in equal units) as used by military engineers. The other half has two irregular scales. One is a polygonal scale marked "anguli centri." It is numbered from 3 to 12, and is designed for laying out the exterior angles of forts with 3 to 12 sides. The other irregular scale is marked "mechanica." It bears the maker’s mark "Jacobus de Steur Fecit Leyde." The bottom drawer of the wooden box contains several auxiliary items: a sighting bar that can fit over the sights on the alidade; a pair of tall sight vanes that can fit over any of the shorter vanes; a head for a jacob's staff; two joints with screw bases (one fixed and the other universal); and a clinometer bob. There are spaces for two other accessories, now lost.
Our collection database is a work in progress. We may update this record based on further research and review. Learn more about our approach to sharing our collection online.
If you would like to know how you can use content on this page, see the Smithsonian's Terms of Use. If you need to request an image for publication or other use, please visit Rights and Reproductions.