Despite the strange shapes of the arms on this container, the fluid inside will always settle to a common height. This is because each interconnected section is open to the outside air. The fluid’s surface has to reach a static equilibrium with the atmosphere–i.e. the surface of the fluid must be at atmospheric pressure–and the pressure at the lowest level in each section must match because the arms are connected. When fluid is added, the height of the columns oscillates some because the momentum of the added fluid carries the column past its equilibrium position, much like a perturbed mass hanging from a spring will oscillate before settling.