Fig 14. Meridian section of a barrel torus. Arc PAP" is circular meridian curve. XX" is axis about which meridian curve PAP" revolves, or axis of revolution. Center of curvature of PAP" is at point C, and R is radius of curvature of meridian curve PAP". r: OA is equatorial radius of curvature, and r1, r2, r3, and r4 are sagittal or transmeridian radii of curvature which are successively longer in order given.