"Jacobus de Steur Fecit Leyde"
11 inches diameter; needle 2.25 inches; sighting bar 11 inches long
Jacob de Steur worked in Leyden in the second half of the 17th century, and 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." 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.