  Using the result of the three flavor analysis of the old Kamiokande data, the
recent Superkamiokande data of atmospheric neutrinos and the CHOOZ reactor
data, it is shown that the third mixing angle $\theta_{13}$ is small. It is
proposed to determine the small value of $\theta_{13}$ and the CP violating
phase $\delta$ in very long baseline experiments by measuring the appearance
probability $P(\nu_\mu\to\nu_e)$ and the T violating effect
$P(\nu_e\to\nu_\mu)-P(\nu_\mu\to\nu_e)$ which are enhanced by the matter effect
of the Earth.
