An instrument for taking heights and distances.
Reach then a soaring quill, that I may write As with a Jacoh's staff to take her height.
The Apostle James is usually represented with a staff.
As he had travelled many a summer's day Through boiling sands of Arabie and You; And in his hand a Jacob 8 staff to stay His weary limbs upon.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894