  Use is made of Onsager's hydrodynamic equation to derive the vibration
spectrum of the vortex lattice in d-wave superconductor. In particular the
rhombic lattice (i.e. the $45^\circ$ tilted square lattice) is found to be
stable for $B>H_{cr}(t)$. Here $H_{cr}(t)$ denotes the critical field at which
the vortex lattice transition takes place.
