  Granular materials partake almost dramatically at times of the properties of
solids and, under different circumstances, of some properties of gases. Here,
within the mechanics of mass points, an elementary analysis, involving
predominantly velocities rather than places, is shown to lead to a global
equation concerning the shuffling motions (in addition to continuity and
Cauchy's equations); it involves a stirring tensor and rules the evolution of a
Reynolds' tensor.
