Water turbine

A sand glass measures time by sand flow through a narrow tube. In contrast, a water clock works on a different principle. The water clock uses a liquid flowing in a medium which is usually also a liquid, but of different density.

The denser liquid displaces the lighter one upwards, similarly to the traditional sand glass where the sand pushes up the air. Naturally, both liquids cannot mix, and, the denser liquid, as contrasted with the medium, is not transparent. Additionally, there is some air in the water clock which travels up together with the liquids in the container.

The turbines move thanks to the falling drops of the dense liquid, as well as thanks to air bubbles that travel upwards. The trick when constructing such a toy is to match the size of the hole through which a liquid or air flows.