40. Cartesian Diver

In a container of water sealed with a rubber diaphragm top is a smaller floating container partially filled with water and with a small hole in the bottom. When the rubber diaphragm is depressed, the air in the smaller container is compressed, increasing the volume of water in the smaller container and reducing its buoyancy. Thus, the smaller container sinks. Its level can be controlled by the finger pressure on the diaphragm.