High Quality Content by WIKIPEDIA articles! High Quality Content by WIKIPEDIA articles! The Whewell equation of a plane curve is an equation that relates the tangential angle with arclength, where the tangential angle is angle between the tangent to the curve and the x-axis and the arc length is the distance along the curve from a fixed point. These quantities do not depend on the coordinate system used except for the choice of the direction of the x-axis, so this is an intrinsic equation of the curve, or, less precisely, the intrinsic equation. If a curve is obtained from another by translation then their Whewell equations will be the same.