Inside U-tube is fluid. Mass of fluid inside tube is m. Fluid density is ρ. Area tube's section is S. Both ends of U-tube are open. In one moment fluid was moved from its equilibrium position by distance Δx. After precipitation from equilibrium position fluid executes harmonic vibrations in tube. Subject of example is to find fluid's harmonic vibrations period. We assume that there is no energy dissipation and harmonic vibrations will last without end.