  We have investigated the thermal Hall effect of magnons for various
ferromagnetic insulators. For pyrochlore ferromagnetic insulators
Lu$_2$V$_2$O$_7$, Ho$_2$V$_2$O$_7$, and In$_2$Mn$_2$O$_7$, finite thermal Hall
conductivities have been observed below the Curie temperature $T_C$ . From the
temperature and magnetic field dependences, it is concluded that magnons are
responsible for the thermal Hall effect. The Hall effect of magnons can be well
explained by the theory based on the Berry curvature in momentum space induced
by the Dzyaloshinskii-Moriya (DM) interaction. The analysis has been extended
to the transition metal (TM) oxides with perovskite structure. The thermal Hall
signal was absent or far smaller in La$_2$NiMnO$_6$ and YTiO$_3$, which have
the distorted perovskite structure with four TM ions in the unit cell. On the
other hand, a finite thermal Hall response is discernible below $T_C$ in
another ferromagentic perovskite oxide BiMnO$_3$, which shows orbital ordering
with a larger unit cell. The presence or absence of the thermal Hall effect in
insulating pyrochlore and perovskite systems reflect the geometric and
topological aspect of DM-induced magnon Hall effect.
