  We report on the experimental observation of solitons propagating along a
torus of fluid. We show that such a periodic system leads to significant
differences compared to the classical plane geometry. In particular, we
highlight the observation of subsonic elevation solitons, and a nonlinear
dependence of the soliton velocity on its amplitude. The soliton profile,
velocity, collision, and dissipation are characterized using high resolution
space-time measurements. By imposing periodic boundary conditions onto
Korteweg-de Vries (KdV) equation, we recover these observations. A nonlinear
spectral analysis of solitons (periodic inverse scattering transform) is also
implemented and experimentally validated in this periodic geometry. Our work
thus reveals the importance of periodicity for studying solitons and could be
applied to other fields involving periodic systems governed by a KdV equation.
