  We investigate the dynamic behavior of a Bose-condensed gas of alkali atoms
interacting with repulsive forces and confined in a magnetic trap at zero
temperature. Using the Thomas-Fermi approximation, we rewrite the
Gross-Pitaevskii equation in the form of the hydrodynamic equations of
superfluids. We present solutions describing large amplitude oscillations of
the atomic cloud as well as the expansion of the gas after switching off the
trap. We compare our theoretical predictions with the recent experimental data
obtained at Jila and MIT.
