Stephens, Alexander, Important Irish Backstaff or Davis quadrant, 1742

Irish, dated to the central stretcher 'made by Alexander Stephens for Mr John Taylor, July the 19th 1742', with boxwood 0-25 degree and 0-60 degree arcs, mahogany frame,

Note: acording to Clifton Alexander Stephens was working in Temple Bar, Dublin from 1745 - 1761 when he went inot partnership with John Alexander until 1781

Backstaffs By Alexander Stephens in museum collections:

• Whipple Museum of the History of Science (University of Cambridge, UK) — Backstaff signed “Made by Alexr. Stephens Temple Barr Dublin For George Kiddrington,” dated 1747. Whipple catalogue ref. noted as Whipple 698. (scientificinstrumentsociety.org)

• Den Antikvariske Samling, Quedens Gaard (Ribe, Denmark) — Backstaff inscribed “MADE BY ALEXANDER [sic] STEPHEN FOR Mr WILLIAM MUSON DUBLIN MAY THE 7th 1732.”

Notes:

• A further Stephens backstaff “Made by Alexander Stephens for Gyles Fearon 1747” is recorded and illustrated in SIS Bulletin 19 (1988), p.21, but that citation is from the “Market-place” section (i.e., a trade listing), not a museum holding.

The Backstaff

The backstaff, or Davis quadrant, was one of the principal instruments used by mariners in the late seventeenth and early eighteenth centuries to determine latitude at sea. It allowed a navigator to measure the altitude of the Sun above the horizon—an essential observation for finding one’s north–south position. The instrument was designed so that the observer faced away from the Sun, using the shadows cast by the upper arc onto a sight vane. By aligning the shadow and the horizon through the sight, the angle between the Sun and horizon could be read on the graduated arc. From this solar altitude, the navigator could calculate the ship’s latitude using tables and the known declination of the Sun for the date (Waters, 1958; Howse, 1993).

This design offered a major advantage over earlier cross-staffs, which required the user to look directly into the Sun, often leading to eye damage and inaccurate readings. The backstaff’s two arcs, the larger 60° arc and a smaller 25° arc, enabled different scales of measurement and improved precision, typically to within half a degree. However, its reliance on shadows meant that it could only be used in bright sunlight, and cloud or haze could easily render it useless (Turner, 1983).

By the 1730s the backstaff was being gradually replaced by the reflecting octant, invented independently by John Hadley in England and Thomas Godfrey in America around 1731. The octant employed mirrors to bring the image of the Sun and the horizon together in a single view. This “double reflection” principle eliminated parallax errors, allowed for greater accuracy, and—crucially—could be used facing the Sun or even by moonlight. The octant’s ability to provide direct sighting, its smaller size, and its improved precision quickly made it the preferred instrument for navigators. By the mid-eighteenth century, backstaffs such as this example were still in service but were rapidly being superseded by the new reflecting technology that would lead eventually to the sextant (Howse, 1993; Bedini, 1994).

References (Harvard format)

• Bedini, S.A. (1994) Thinkers and Tinkers: Early American Men of Science. New York: Charles Scribner’s Sons.

• Howse, D. (1993) Greenwich Time and the Longitude. London: Philip Wilson Publishers.

• Turner, G.L’E. (1983) Scientific Instruments, 1500–1900: An Introduction. London: HMSO.

• Waters, D.W. (1958) The Art of Navigation in England in Elizabethan and Early Stuart Times. London: Hollis & Carter.

Note: For this specific lot, 5% import VAT is applicable on the hammer price.