The reaction principle
Water flows into the centre and out of bent arms at the ends. The back-pressure (reaction) turns the wheel in the opposite direction. It is a demonstration of Newton's law of action and reaction, which rocket engines also rely on.
A scientist of many fields
Segner studied medicine, was a professor of mathematics and physics, and researched the surface tension of liquids as well as the rotation of bodies. A crater on the Moon is named in his honour.
Legacy
Segner's wheel is the forerunner of the water turbines that produce electricity in hydroelectric power stations today. The theory of turbines was later greatly developed by another Slovak, Aurel Stodola.