Key points are not available for this paper at this time.
A ``natural time'' is defined in terms of the viscometric functions of a simple fluid. Its magnitude is a measure of the relative importance of normal-stress effects. Such a time is shown to be an ordering parameter for departures from the theory of linear viscosity in fluids with fading memory. The frequency dependence of the absorption and phase shift in the diffusion of forced sinusoidal shearing at the boundary of a semi-infinite mass of fluid are shown to depend critically upon the natural time.
C. Truesdell (Sat,) studied this question.