\Gy"ro*scope\, n. [Gr. ? ring, circle + -scope.]
1. A rotating wheel, mounted in a ring or rings, for
   illustrating the dynamics of rotating bodies, the
   composition of rotations, etc. It was devised by Professor
   W. R. Johnson, in 1832, by whom it was called the
   {rotascope}.
2. A form of the above apparatus, invented by M. Foucault,
   mounted so delicately as to render visible the rotation of
   the earth, through the tendency of the rotating wheel to
   preserve a constant plane of rotation, independently of
   the earth's motion.