- Spinning navigational aid - 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. - 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.