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Heron's Fountain is a mind bending hydrodynamic mechanism. It appears to be perpetual motion but it fact it's a really clever mix of pneumatics and hydraulics. This 2D presentation helps to explain how it works.

Image credits:

Soxhlet extractor image Vivi! (https://commons.wikimedia.org/wiki/Fi...)
Washout toilet Flammingo (https://commons.wikimedia.org/wiki/Fi...)
Toilet diagrams SouthHamsian (https://en.wikipedia.org/wiki/File:Fo...)

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