  Using a sample of $771.6 \times 10^{6}$ $\Upsilon(4S)$ decays collected by
the Belle experiment at the KEKB $e^+e^-$ collider, we observe for the first
time the transition $\Upsilon(4S) \to \eta h_b(1P)$ with the branching fraction
${\cal B}[\Upsilon(4S) \to \eta h_b(1P)]= (2.18 \pm 0.11 \pm 0.18) \times
10^{-3}$ and we measure the $h_b(1P)$ mass $M_{h_{b}(1P)} = (9899.3 \pm 0.4 \pm
1.0)$ MeV/$c^{2}$, corresponding to the hyperfine splitting $\Delta M_{\mathrm
HF}(1P) = (0.6 \pm 0.4 \pm 1.0)$ MeV/$c^{2}$. Using the transition $h_b(1P) \to
\gamma \eta_b(1S)$, we measure the $\eta_b(1S)$ mass $M_{\eta_{b}(1S)} =
(9400.7 \pm 1.7 \pm 1.6)$ MeV/$c^{2}$, corresponding to $\Delta M_{\mathrm
HF}(1S) = (59.6 \pm 1.7 \pm 1.6)$ MeV/$c^{2}$, the $\eta_b(1S)$ width
$\Gamma_{\eta_{b}(1S)} = (8 ^{+6}_{-5} \pm 5)$ MeV/$c^{2}$ and the branching
fraction ${\cal B}[h_b(1P) \to \gamma \eta_b(1S)]= (56 \pm 8 \pm 4) \%$.
