At the left the Hero's fountain, based on the air compression and on the water incompressibility.
The water comes into the bottle 2 and forces the air to enter the bottle 1.
The air entrance forces the water to exit gushing trought the tube 3.

Hero is also celebrated for other amazing devices, one of which powered 'self-opening doors' of a temple.
Lighting a fire caused the air beneath the altar to warm and expand, increasing the pressure in B, a water reservoir below (see figure at the right).
This forced water through a siphon (S) into a hanging bucket .
The descent of the bucket pulled on ropes which opened the doors.
When the fire was extinguished, the water returned into the vessel and the counter-weight (W) shut the doors.