Double cylindrical calipers. Used to find thickness of cannons, pipes, etc. Two circular points and 2 squared points on opposite sides of central turning plate. The curved pieces are of equal length and meet on a central line. The squared pieces meet off-center, as one has a longer bottom piece and the other a shorter piece. By putting the squared ends one inside and one outside the barrel of a cannon, the separation of the curved ends will indicate the thickness of the barrel. Can also be used to find the diameter of spherical objects. Completely covered with floral and other decorative engraving.
