Water flows out of a large cylindrical tank through a long and thin horizontal pipe fitted at the bottom of the tank. There are five thin vertical glass pipes fitted on the horizontal pipe at equal intervals.

Pressures and pressure differences can be determined with the help of the level of fluids in the tank and the vertical pipes. We imagine a (closed) hydraulic circuit (a path) going from the top of the tank down to the horizontal pipe, then through the pipe and back through the air to the top of the tank. Pressure differences along this path can be determined from the data. It turns out that the sum of all pressure differences is equal to zero along the closed path.