  We report the experimental observation of a robust horizontal mean flow
induced by internal gravity waves. A wave beam is forced at the lateral
boundary of a tank filled with a linearly stratified fluid initially at rest.
After a transient regime, a strong jet appears in the wave beam, with
horizontal recirculations outside the wave beam. We present a simple physical
mechanism predicting the growth rate of the mean flow and its initial spatial
structure. We find good agreement with experimental results.
