The Heron’s ball is a bellied vessel, which is partly filled with water and sealed with a cork. A pipe which is open on both the sides goes inside the vessel through a hole in the cork. When the air pressure inside the vessel becomes higher than the atmospheric pressure, the water pushes out through the pipe. The internal air pressure can be increased in different ways: by connecting two vessels, by blowing air in the vessel through another pipe, or by heating the water. The water vapour that is formed needs more space and pushes water out from the pipe; the water actually sprays out. In principle, the Heron’s ball is the first steam engine.