It was a conception of Philon of Byzantium depicting a dragon or a deer that drank water with great hunger and roared only when Pan turned away from it. When Pan turned, the dragon relaxed. The theme was repeated continuously.

For the operation of the automaton, water from a spring was driven inside the airtight intermediate container forcing air to leave towards the built-in tube of the dragon or the deer giving a roaring impression. Then, when the water level exceeded the axial siphon, it emptied through it to the lower container, therefore, because of the vacuum created by the emptying of the container, the tube incorporated in the deer sipped water and air giving the drinking impression. If Pan turned to the deer, the water from the spring was driven by rotating horizontal pipe directly into the lower container, the phenomenon ceased and the water level in the large crater (bowl) rose.

SOURCES: Heron of Alexandria, Pneumatics, A30′, ‘Philon of Byzantium, Pneumatics, 59.