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An Unusually Fine Example Of An
Early American Backstaff
HAM, James. Backstaff, fruitwood frame with two boxwood arcs. 1759.
[58.5 cm long], Smaller ivory lozenges engraved with geometric patterns. The larger arc [35 cm] stamped with makers’ initials and “No. 151”, and incised with scale reading 0-25° flanked by fleur-de-lys patterns; the smaller arc [20 cm] incised with scale 0-60° in 1° increments. A set of diagonals and three later vanes enable readings to be taken to 10 minutes. Longer arc slightly worn and darkened at lower end; but generally in excellent state of preservation.
$22,500 Rare, early American-made back-staff, one of the principle
instruments of navigation throughout the 17th and 18th centuries. According
to Deborah Jean Warner of the NMAH, the back-staff was the first
navigational instrument, and probably the first mathematical instrument of
any kind, produced in large numbers in America. Used to measure latitude at
sea, it was a marked improvement on the traditional cross-staff, which
required navigators to stare directly into the sun and could cause
blindness.
The backstaff-also called the Davis quadrant because the two arcs together total 90 degrees-was designed in the early 1590s by the Welsh sea captain John Davis during his voyage to search for the Northwest Passage. Davis hoped to overcome the problems he had with glare when taking sights in the Arctic with an astrolabe or cross-staff. His two-arc design simplified the sighting process by allowing the observer to view both the horizon and the shadow of the sun on the horizon vane simultaneously.
The backstaff consists of two triangular arcs, the larger calibrated to 30° and the one at the apex of the instrument calibrated to 60°. Measurements are taken by casting the shadow of the sun onto the degree scale. In his 1795 <Mathematical and Philosophical Dictionary, Charles Hutton provides detailed instructions: "To use the Back-Staff..., the observer turns his back to the sun, lifts up the instrument, and looks through the sight-vane, raising or falling the quadrant, till the shadow of the upper edge of the shade-vane fall on the upper edge of the slit in the horizon-vane; and then if he can see the horizon though the said slit, the observation is exact, and the vanes are right set" (p. 191). The precise latitude is determined by adding together the degrees marked on the arcs, and then referring to a table.
Most backstaffs were produced in England, but a few Dutch, Irish and American examples are known. Typically constructed from economical fruitwood, a great deal of care was lavished on shaping the instruments' components to avoid warping and to ensure the accuracy of the scales. The inlaid ivory plaques and fine detailing on the utilitarian apparatus reflect the careful craftsmanship.
James Ham advertised a full range of mathematical instruments and nautical compasses for sale first in New York (1754), and then in Philadelphia (1761). While Ham's original business address was the Widow Ratsey's house near the old Dutch Church on Smith Street in New York, this backstaff was evidently produced at his new residence, "at the sign of Hadley's Quadrant at Front and Water Streets, Philadelphia, Pa." An important shipping center during the 18th century, Philadel-phia was consequently one of the primary markets for nautical instruments. Ham's son James Jr. took over the business by 1780, and according to his advertisements, also specialized in "Hadley and Davis Quadrants."
* S. Bedini, Early American Scientific Instruments and their Makers (Washington, 1964), pp. 65-66; Charles Hutton, Mathematical and Philosophical Dictionary (1795), p. 191.
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