Heron's fountain is a hydraulic machine invented by the 1st century AD inventor, mathematician, and physicist Heron of Alexandria (also known as Hero of Alexandria).
Heron studied the pressure of air and steam, described the first steam engine, and built toys that would spurt water, one of them known as Heron's fountain. Various versions of Heron's fountain are used today in physics classes as a demonstration of principles of hydraulics and pneumatics.
In the following description, call the 3 containers:
- (A) Top: basin
- (B) Middle: water supply
- (C) Bottom: air supply
And three pipes:
- P1, (on the left in the picture), from a hole in the bottom of basin (A) to bottom of air supply container (C)
- P2, (on the right in the picture), from the top of the air supply container (C) to the top of the water supply container (B)
- P3, (in the middle of the picture), from the bottom of the water supply container (B), up through the bottom of the basin (A) to a height above the basin's rim. The fountain will issue upwards through this pipe. The maximum height of P3 pipe depends on the height between B and C (see below).
It is okay if A is closed and airtight, but it doesn't need to. B and C, however, need to be airtight, and resistant to atmospheric pressure. Plastic bottles will do, but a glass container will do better; balloons will just not work. This is because the fountain works the following way:
- the energy for the running of water ultimately comes from the water in B descending into C
- this means the water in B can rise into A only as much as it falls from B to C
- water falling from A down to C through pipe P1 will build up pressure in the bottom container; this pressure is equal to the height difference between A and C
- pressure is transmitted by the air through pipe P2 into the water supply B, and push up the water into pipe P3
- water up pipe P3 replace water falling from A into C, closing the loop.
These principles explain the construction:
- We don't want the air in C escape through pipe P1, which is why it must go to the bottom so that water seal it
- We don't want the air in B to escape through pipe P3 either, which is why it must go to the bottom so that water seal it
- We must prevent water to go directly through pipe P2 from B to C, so it must connect the top of B with C
- if P2 were connected in C at the bottom of it, water would seal it and air pressure would not build up in B. So P2 must be connected at the top of C
Heron's fountain is not a perpetual motion machine. If the nozzle of the spout is narrow, it may play for several minutes, but it eventually comes to a stop. The water coming out of the tube may go higher than the level in any container, but the net flow of water is downward. If, however, the volumes of the air supply and fountain supply containers are designed to be much larger than the volume of the basin, with the flow rate of water from the nozzle of the spout being held constant, the fountain could operate for a far greater time interval.
Its action may seem less paradoxical if considered as a siphon, but with the upper arch of the tube removed, and the air pressure between the two lower containers providing the positive pressure to lift the water over the arch. The device is also known as Heron's siphon.
The gravitational potential energy of the water which falls a long way from the basin into the lower container is transferred by pneumatic pressure tube (only air is moved upwards at this stage) to push the water from the upper container a short way above the basin.
The fountain can spout (almost) as high above the upper container as the water falls from the basin into the lower container. For maximum effect, place the upper container as closely beneath the basin as possible and place the lower container a long way beneath both.
As soon as the water level in the upper container has dropped so low that the water bearing tube no longer touches the water surface, the fountain stops. In order to make the fountain play again, the air supply container is emptied of water, and the fountain supply container and the basin are refilled. Lifting the water provides the energy required.
Reiterative motion and variants
As previously mentioned, the fountain will stop working when water from B has dropped to C. There are ways, however, to make it work again, such as:
- design piping so that once C is full and B is empty, their position can be switched.
- add valves to empty C and replenish B (in effect transferring water from C to B).
- instead of using valves, transfer water up from C to B through boiling and condensing.
- make a 4 container A-B-C-D fountain, which can be turned upside down so that the full and empty container switch places.
There also exist fountains with two liquids of different colors and density.
It is possible that geysers operate via this mechanism, with the distinction that the spouting of the water at the surface occurs discontinuously. Furthermore, unlike Heron's fountain, which requires that the air supply container be manually emptied of water, geysers have an analogous "air supply container" that is steadily heated by geothermal energy. When the water level in the air supply container becomes too high, the geothermal heat flux causes the water to boil off and therefore naturally empty the container of water and replace it with water vapor instead of air.
In popular culture
- physics.kenyon.edu; Hero's Fountain
- Brown, Henry T.; "507 Mechanical Movements, Mechanisms and Devices", p. 111; 19th edition 1901.
- Hiscox, Gardner D.; "1800 Mechanical Movements, Devices and Appliances", p. 162; 16th edition published 1926 under the name "Mechanical Movements, Power and Devices".
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