A @S05534@ method based on the fact that at the meniscus and at the bottom of the centrifuge cell there is never a flux of the solute across a plane perpendicular to the radial direction and the equations characterizing the @S05536@ always apply there, even though the system as a whole may be far from equilibrium. The use of the term 'approach to @S05536@' for Archibald's method is discouraged, since it has a more general meaning.