A wooden backstaff (or Davis quarter) unsigned, composed of two arcs: the sight vane arc is numbered 0-25° and 65-90°, the smaller arc is divided into 62°.

Facing the Sun, it indicates its height above the horizon, but the originality of the instrument was to obtain the same result using it backwards. The lens added on the sliding sight of the little arc was imagined around 1770 in order to be used in case of cloudiness.

Under optimum conditions the accuracy was up to 1/10th of a degree!

Literature: « Marine Antiques – a Chronicle of a Unique Collection – Dominique Delalande –New York 1986 » The calculation of latitude pp 2 & 3. « L’instrument de marine » – Jean Randier p 119.

NB: nearly always missing, the vane and sliding sights in boxwood have been replaced and recalculated later.

Dimensions: length 65 cm – width 37 cm.

England 1750/1760.

Reference : D/TA

Price on request.