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Here the two centers of curvature are not the same, and the one road narrows to the other on the curve.

The centers of curvature of the different rings receded from the axis as their distance from the center increased, in such a manner as to overcome a serious optical defect known as spherical aberration.

To find the center of curvature at D, assume the crank pin A to have a velocity Aa.

To run round a curve, the axis of each must, if continued, pass through the center of curvature of the curve.