We study, using simulations, the steady-state flow of dry sand driven by gr
avity in two dimensions. An investigation of the microscopic grain dynamics
reveals that grains remain separated but with a power-law distribution of
distances and times between collisions. While there are large random grain
velocities, many of these fluctuations are correlated across the system and
local rearrangements are very slow. Stresses in the system are almost enti
rely transfered by collisions and the structure of the stress tensor comes
almost entirely from a bias in the directions in which collisions occur. [S
1063-651X(99)14903-1].