  Large amplitude oscillations of solar filaments is a phenomenon known for
more than half a century. Recently, a new mode of oscillations, characterized
by periodical plasma motions along the filament axis, was discovered. We
analyze such an event, recorded on 23 January 2002 in Big Bear Solar
Observatory H$\alpha$ filtergrams, in order to infer the triggering mechanism
and the nature of the restoring force. Motion along the filament axis of a
distinct buldge-like feature was traced, to quantify the kinematics of the
oscillatory motion. The data were fitted by a damped sine function, to estimate
the basic parameters of the oscillations. In order to identify the triggering
mechanism, morphological changes in the vicinity of the filament were analyzed.
The observed oscillations of the plasma along the filament was characterized by
an initial displacement of 24 Mm, initial velocity amplitude of 51 km/s, period
of 50 min, and damping time of 115 min. We interpret the trigger in terms of
poloidal magnetic flux injection by magnetic reconnection at one of the
filament legs. The restoring force is caused by the magnetic pressure gradient
along the filament axis. The period of oscillations, derived from the
linearized equation of motion (harmonic oscillator) can be expressed as
$P=\pi\sqrt{2}L/v_{A\phi}\approx4.4L/v_{A\phi}$, where $v_{A\phi}
=B_{\phi0}/\sqrt{\mu_0\rho}$ represents the Alfv\'en speed based on the
equilibrium poloidal field $B_{\phi0}$. Combination of our measurements with
some previous observations of the same kind of oscillations shows a good
agreement with the proposed interpretation.
