Liouville - type theorems for - subharmonic functions

A viscous damping device positioned between a gimbal shaft and a bearing sleeve supporting the gimbal shaft is provided for damping a gyro apparatus in a rotational direction. The viscous damping device comprises first and second rings fixed to an outer periphery of the gimbal shaft. A circular groove provided in the first ring is complementary with a circular protrusion of the second ring, so that damping liquid initially poured into the circular groove is made to fill an air gap between the outer peripheries of the rings and an inner wall of the bearing sleeve by inserting the circular protrusion into the circular groove.