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An instrument which could more readily adapt itself to the swaying of the observer's body in a sea-way, soon displaced in good measure the astrolabe on shipboard. It in several ways modified forms for a long time served mariners as a convenient help in ascertaining the altitude of the celestial bodies. Precisely when it was first introduced is not certain; but the earliest description of it which has been found is that of Werner in 1514.

Jackstaff

An instrument which could more readily adapt itself to the swaying of the observer's body in a sea-way,…