  A two-layer fluid system separated by a pycnocline in the form of an internal
wave is considered. The lower layer is infinitely deep, with a higher density
than the upper layer which is bounded above by a flat surface. The fluids are
incompressible and inviscid. A Hamiltonian formulation for the fluid dynamics
is presented and it is shown that an appropriate scaling leads to the
integrable Benjamin-Ono equation.
