Although the hydraulic clock that was inside the tower has not been saved, it can be reconstructed based on the holes and grooves that exist in its floor and in the walls of the cylindrical turret. Notches and archeological members accurately indicate a cylindrical railing for the protection of the clock and three recesses for the bases of the statues that supported its mechanism.
The elevated marble semi-cylindrical tank of the turret was filled with water through an underground metal tube at the top. A little lower, a second metal pipe functioned as an overflow, achieving a constant level and at the same time feeding two drinking water fountains and the fountain of a large copper or marble basin in the center of the floor. A little above the bottom of the tank, a third metal tube fed a copper cylindrical tank (via a suitable nozzle) therefore with a simultaneous outflow of water. The clock was probably of the “referential” type, ie with an eccentric drum that rotated behind a brass grid that set the hours based on the “ascension”. The rotation of the index – drum was achieved through a pulley and a chain with a counterweight and a weight-float that rose slowly due to the simultaneous rise of the water level in the copper cylindrical tank. The watch could operate automatically and uninterruptedly without human intervention with the appropriate hydraulic and mechanical device (curved pipette, pulley, pawl (ratchet) and gears with gear ratio 364/365). Thus during a 24 hour period the pointer performed a complete rotation indicating the 12 day and 12 night asynchronous hours (depending on the season) while the drum was delayed by 1/365 to accurately indicate the schedule of the next day. At the end of every 24 hours the water exceeded the curved siphon built into the copper tank and emptied quickly. The float weight, the chain and the counterweight were returned to their original position (while the drum and the pointer were stationary due to the pulley pawl) to restart the process. There is a very high probability that due to the importance and richness of the monument in the hydraulic clock, the automatic indication of the Athenian calendar and the cycles of Meton and Callipus were attached with the appropriate mechanical devices (similar to those found in the Antikythera mechanism). The positions of the sun and the planets (Mercury, Venus, Mars, Jupiter and Saturn) but also the position and phase of the moon.