| Instrument | : | Davis Quadrant or Backstaff |
| Manufacturer | : | N. de Hilster (original by W. Garner). |
| Country of origin | : | The Netherlands (Original: U.K.) |
| Manufacturing year | : | 2003 (Original: 1734) |
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 invented by Captain John Davis and first described in his Seamen's Secrets published in 1595.
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 eliminated by the invention of the 'Dutch shoe' by Dutch sailors in the 18th 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 the instrument got it's name Backstaff. The instrument is capable of measuring an angle of 90 degrees and therefor called a quadrant (1/4th of a circle).
Materials used are a combination of Cambara (cultivated tropical hardwood from Brasil: Erisma Uncinatum or Erisma Lanceolatum of the Vochysiaceae family) for the frame and cherry wood for the arcs or ebony for the frame and boxwood for the arcs. 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 therefor 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 engrave and therefor is very suitable for the scales.
On October 11th, 2005 and November 5th, 2007 we conducted field tests in which the Davis Quadrant was involved. The first test was done with instrument number 3, the second with number 4. Both tests showed that the Davis Quadrant tends to measure too high, unless the upper limb is measured. The first test, where the lower limb was measured showed an average error of +11.5 arc minutes, with a standard deviation of 8.7 arc minutes. The second test showed an average error of +10.5 arc minutes with a standard deviation of 5.1 arc minutes (calculated over the second part of the trial, see graph).
I'm very grateful to Peter Ifland and Willem Mörzer Bruijns as without their help and information (I was even allowed to study an 1803 original) the detailed reproduction of this instrument would never have been possible.
If you like to know more, don't hesitate to contact me.