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.

When the water does not flow, the levels of liquid are the same in the tank and in all five vertical pipes. As soon as the water flows, the levels in the vertical galss pipes decrease (the change is very quick at the beginning, and very slow later on; the slow change parallels the slow decrease of the level in the large tank).

The water levels in the five vertical pipes decrease linearly in the direction of flow through the horizontal pipe.