  We investigate the energy transfer from large waves to small ones through
vertical acceleration and demonstrate that this is a much larger effect than
that of the potential energy changes of the small waves moving over the larger
ones. Rates of exponential growth for this process are given and limits on the
stable size of small waves in the horizontal accelerations from the larger ones
are derived. We discuss the possibility of this being a manifestation of the
Benjamin-Feir instability.
