Note:  Do not rely on this information. It is very old.

Cartesian Devil

Cartesian Devil, or Cartesian Diver, called also the Bottle Imp, is a mechanical toy which illustrates atmospheric pressure. It consists of a hollow figure having a hole near the top, and partly filled with air and partly with water. This is partially immersed in water contained in a wide-mouthed vessel, the opening of which is covered with indiarubber or other elastic material. If pressure be applied to this cover, the air in the figure is compressed, and water enters to compensate, and the figure sinks, to resume its former position when the pressure is removed.