Instantaneous Axis of a Cylinder rolling on a Cylinder.Let a cylinder bbb, whose axis of figure is B and angular velocity -y, roll on a fixed cylinder **acm**, whose axis of figure is A, either outside (as infig.

The whole of the foregoing reasonings are applicable, not merely when **acm** and bbb are actual cylinders, but also when they are the osculating cylinders of a pair of cylindroidal surfaces of varying curvature, A and B being the axes of curvature of the parts of those surfaces which are in contact for the instant under consideration.