cross-staff

English

cross-staff (plural cross-staffs or cross-staves)

(ecclesiastical, obsolete) A archbishop's cross.
(historical) An early instrument, consisting of a wooden rod with a sliding crosspiece, used for measuring the altitude of a star or for surveying.
- 2015, David Wootton, The Invention of Science, Penguin, published 2016, page 188:
  The cross-staff is a very simple instrument, a calibrated shaft along which a bar slides.