The Collaborative International Dictionary of English v.0.48:

Forestaff \Fore"staff`\, n. (Naut.)
   An instrument formerly used at sea for taking the altitudes
   of heavenly bodies, now superseded by the sextant; -- called
   also cross-staff. --Brande & C.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cross-staff \Cross"-staff`\ (-st?f`), n.
   1. An instrument formerly used at sea for taking the
      altitudes of celestial bodies.
      [1913 Webster]

   2. A surveyor's instrument for measuring offsets.
      [1913 Webster]