A convex sphere set in a concave hemisphere, the two having a common center, the one fitting in the other as shown in Figure 1, can be rotated on any one of its diameters, and only on a diameter, as an axis. The center of curvature, the point to which all the radii converge, is the center of rotation. The point on the surface at which the force is applied and the direction of the force determine what the plane of rotation shall be and fix the axis of rotation at right angles to this plane. This can be best understood by a study of Figure 1.
A B D represents the concave hemisphere whose center is c; a d b e represents the convex sphere whose center is also c: If the force is so applied that a b shall be the rotation plane, the axis of rotation will