The Greeks first invented a working alarm clock around 250 BC, when they created a water clock, also known as a "clepsydra."
In the Drum Tower incense-sticks, specially prepared by the astronomical board, are kept burning to mark the passage of time, in which important duty their accuracy is checked by a clepsydra.
The various approaches to the citadel on the northern side - the rock-cut flight of steps north-east of the Erechtheum, the stairs leading to the well Clepsydra, and the intermediate passage supposed to have furnished access to the Persians - are all to be attributed to the primitive epoch.
The Pelasgic wall enclosed the spring Clepsydra, beneath the north-western corner of the Acropolis, which furnished a watersupply to the defenders of the fortress.
On the north-west rock the caves known as the grottoes of Pan and Apollo were cleared out; these consist of a slight high-arched indentation immediately to the east of the Clepsydra and a double and somewhat deeper cavern a little farther to the east.