  We show that the equation of quasigeostrophic (QG) potential vorticity
conservation in geophysical fluid dynamics follows from Hamilton's principle
for stationary variations of an action for geodesic motion in the f-plane case
or its prolongation in the beta-plane case. This implies a new momentum
equation and an associated Kelvin circulation theorem for QG motion. We treat
the barotropic and two-layer baroclinic cases, as well as the continuously
stratified case.
