A rotor (1) is of a substantially spherical shape with one complete half of the sphere (2) still remaining. The rotor (1) effectively takes the shape of the head of a "mushroom". The other half of the sphere has had its material removed or omitted forming the stem (3). The depth of the cut "D" and the length "L", the radius of the sphere, will determine the ratio of the respective weights of the complete half sphere (2) and the stem portion (3). Suitably the weight of the one half to the other half of the spherical body is between .75 and .85 and more preferably .8. When an external force is applied to the rotor (1) it rotates about a pair of orthogonal axes one of which is defined by the edge (6) of the spherical body between the two halves.
