  We present results from a series of experiments on a granular medium sheared
in a Couette geometry and show that their statistical properties can be
computed in a quantitative way from the assumption that the resultant from the
set of forces acting in the system performs a Brownian motion. The same
assumption has been utilised, with success, to describe other phenomena, such
as the Barkhausen effect in ferromagnets, and so the scheme suggests itself as
a more general description of a wider class of driven instabilities.
