1757 VAN KEULEN II, J. - RESTORED CROSSTAFF SEGMENT

RESTORED CROSSTAFF SEGMENT, Dutch, 1757, signed “I*v*K” for Johannes van Keulen, dated “1757” and numbered “23.” The staff is made of ebony, 11/16” (18 mm) square in cross section. The original 12-1/2” (32 cm) long mid-18th century segment is ruled with nonlinear scales on all four sides, each side numbered. This segment is the crucial eye end, with its squared off shape, name and date, and scale origins.
The rest of the staff, with continuations of the existing scales, was lost at some time.
All the scales are hand divided; the numerals, letters, and decorative stars are all punched. The graduations are in degrees of altitude, and with corresponding reversed scales of zenith distance. 

 Condition is very fine, noting some small chips. This segment has now been extended to the correct length of 29-1/2” (750 mm), using a plain ungraduated shaft of black Gaboon ebony.

The replacement vanes are made of pearwood (and bone) with brass fittings — with overall lengths of 3-1/2”, 6-3/4”, 13-3/8”, and 20” (4” for the cross wise bone). These lengths are determined by the scale construction on the staff, and are seen to fall very close to the “desired” ratio of 1:2:4:6:9 (9 being the staff length).
The vanes are correct aesthetically as well as technically, made to the specifications of Willem Morzer Bruyns, author of The Cross Staff. Their design is based in detail on one existing set of original van Keulen vanes, but adjusted for the exact scale lengths of the present instrument (and thus subtending the correct angles when in use). 

Also copied are the rare removable brass aperture disk (sometimes called a “Dutch shoe”), and the fourth (the shortest) vane termed the “horizon vane,” itself fitted with a bone vane. These devices permitted “backward” observations with the cross-staff, thus saving the navigator’s eyes (see Morzer Bruyns). 

 The vanes are accurate in all details, down to the butterfly-shaped thumbscrews, the hand-cut threads, the tapered dovetailed brass fittings, the pressure plates, the construction of each vane from a single piece of pearwood, etc.

Morzer Bruyns’ 1994 crosstaff inventory shows only 95 known worldwide, with very few in private hands. The present example would seem to be very close to inventory #66, which is similarly constructed, signed, and decorated (using three stars before and after the initials and the date), and which is dated 1758.
The maker of the present instrument was the famous firm of Johannes van Keulen of Amsterdam, important maker of cross-staffs and official supplier to the VOC, the Dutch East India Company.