  Apparent mass measurements at the bottom of granular packings inside a
vertical tube in relative motion are reported. They demonstrate that Janssen's
model is valid over a broad range of velocities v. The variability of the
measurements is lower than for static packings and the theoretical exponential
increase of the apparent mass with the height of the packing is precisely
followed (the corresponding characteristic screening length is of the order of
the tube diameter). The limiting apparent mass at large heights is independent
of v and significantly lower than the static value.
