In a U-tube, oil of density 800 kg m^-3 in one arm balances a water column in the other. If the oil column is 10 cm, the balancing water column is:

Since the same volume of fluid must pass every cross-section per second (continuity), the fluid speeds up where the pipe narrows; Bernoulli's equation then says this faster-moving fluid has lower pressure - the principle behind a venturi meter and an aircraft wing's lift.