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Related to cross-staff: Jacob's staff
A forerunner of the modern sextant, used for measuring altitudes of celestial bodies, consisting essentially of a wooden rod with one or more perpendicular crosspieces free to slide along the main rod. Also known as forestaff; Jacob's staff.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.