1734 Will Garner Davis Quadrant (reproduction)In 2002 I obtained drawings of a 1734 Backstaff or Davis Quadrant (Instrument made by Will Garner for Oliver Thompson) with the intention to make this replica.
The Davis Quadrant was the successor of the cross-staff and the predecessor of the octant. The instrument was based on a backstaff invented by Captain John Davis which he first described in his Seamen's Secrets published in 1595. The first time the double arced shape was shown was in George Waymouth's 1604 manuscript 'The Jewell of Artes'.
More on the early development of the instrument (between 1595 and 1680) will be dealt with by me during the A Sense Of Direction symposium on May 6th and 7th, 2010, at the National Maritime Museum in Greenwich, UK, and published thereafter in an article and partially on this web page.1
The advantage of this instrument over the cross staff is that an observer doesn't need to observe the sun with the bare eye (a disadvantage that was overcome by the invention of the brass aperture disc in the 17th century). Instead one can observe it's shadow (or image when the lens vane was used) while standing with his back towards it. This is how instruments based on this principle, like the Davis' original instruments, the Davis Quadrant, the demi-cross and the hoekboog (double triangle) got their general name backstaff. The instrument is capable of measuring angles up to 90 degrees and therefore called a quadrant (1/4th of a circle).
Materials used for the first two attempts were Cambara (cultivated tropical hardwood from Brasil: Erisma Uncinatum or Erisma Lanceolatum of the Vochysiaceae family) for the frame and cherry wood for the arcs. Later replicas were made of ebony for the frame and boxwood for the arcs (see figure 3 and figure 4).
The reason for using two kinds of wood is simple: The tropical hardwood has a very straight grain, is tough and quite resistant to changes in temperature and humidity and therefore will give a very sturdy frame. Another advantage of the hardwoods like ebony is that they make the instrument heavier, resulting in a more stable instrument and better observations. Fruit woods like the cherry wood have a very fine grain that allows it to be engraved finely and therefore very suitable for the scales.
The diagonal scale (see figure 8) on the instrument was introduced around 1660 and first described in detail by John Seller in 1669 in his Practical navigation. About it he wrote that '… possibly this manner of division may not be understood by every one that has occasion for this Instrument, for their sakes therefore have annexed this following Figure.' and it was accompanied by a figure of a section of a diagonal scale.
How the diagonal scale was engraved on the instrument is not explained in period literature, but from research I did on Davis Quadrants by Will Garner in the collection of the National Maritime Museum in Greenwich it became apparent that some sort of dividing engine was used in the process, even though they were produced almost half a century before Jesse Ramsden created his first dividing engine. The research showed an 83% correlation between the two instruments. I too use a dividing aid to create the scales on my Davis Quadrants, but they show less correlation, which is probably caused by a combination of (my lack of) experience and the quality of the dividing engines used (his being more stable than mine).
Notes N. de Hilster, 'The Early Development of the Davis Quadrant', in: Bulletin of the Scientific Instrument Society No. 110, (2011), pp.14-22.
If you have any questions and/or remarks please let me know.
1580s Mariner's astrolabe 1590 Hood's cross-staff 1618 Demi-cross 1623 hoekboog 1660 spiegelboog 1661 Kronan cross-staff 1720 Hasebroek cross-staff 1734 Davis quadrant Early 19th c. ebony octant Late 19th c brass octant 1941 U.S. Navy quintant Hirado navigation set