The backstaff was developed by an English Captain named John Davis in the
1590s and–as it enabled a navigator to observe the sun’s altitude with
safety and ease–was soon widely used for determining latitude at sea. Some
English texts refer to it as Davis’ quadrant, while some Continental European
texts refer to it as the English quadrant.
Ref: J. A. Bennett, The Divided Circle. A History of Instruments for
Astronomy, Navigation and Surveying (Oxford, 1987), pp. 35-36.
Deborah Warner, "Davis’ Quadrants in America," Rittenhouse
3 (1988): 23-40.