  We report experimental measurements of the flow in a cubic container
submitted to a longitudinal libration, i.e. a rotation modulated in time.
Velocity fields in a vertical and a horizontal plane are measured in the
librating frame using a corotating particle image velocimetry system. When the
libration frequency $\sigma_0$ is smaller than twice the mean rotation rate
$\Omega_0$, inertial waves can propagate in the interior of the fluid. At
arbitrary excitation frequencies $\sigma_0<2\Omega_0$, the oscillating flow
shows two contributions: (i) a basic flow induced by the libration motion, and
(ii) inertial wave beams propagating obliquely upward and downward from the
horizontal edges of the cube. In addition to these two contributions, inertial
modes may also be excited at some specific resonant frequencies. We
characterize in particular the resonance of the mode of lowest order compatible
with the symmetries of the forcing, noted [2,1,+]. By comparing the measured
flow fields to the expected inviscid inertial modes computed numerically
[L.R.M. Maas, Fluid Dyn. Res. \textbf{33}, 373 (2003)], we show that only a
subset of inertial modes, matching the symmetries of the forcing, can be
excited by the libration.
