  Based upon spin-lattice interaction, we propose a theoretical model for the
phonon Hall effect in paramagnetic dielectrics. The thermal Hall conductivity
is calculated by using the Kubo formula. Our theory reproduces the essential
experimental features of the phonon Hall effect discovered recently in ionic
dielectric Tb$_3$Ga$_5$O$_{12}$, including the sign, magnitude and linear
magnetic field dependence of the thermal Hall conductivity.
