The Collaborative International Dictionary of English v.0.48:

Compasses \Com"pass*es\, n., pl.
   An instrument for describing circles, measuring figures,
   etc., consisting of two, or (rarely) more, pointed branches,
   or legs, usually joined at the top by a rivet on which they
   move.
   [1913 Webster]

   Note: The compasses for drawing circles have adjustable pen
         points, pencil points, etc.; those used for measuring
         without adjustable points are generally called
         dividers. See Dividers.
         [1913 Webster]

   Bow compasses. See Bow-compass.

   Caliber compasses, Caliper compasses. See Calipers.

   Proportional, Triangular, etc., compasses. See under
      Proportional, etc.
      [1913 Webster]