It was a most brilliant hydraulic clock which indicated, with accuracy, the 365 different hours of the year.
A cam disc – on which a drawing represented the sky and the zodiac cycle – rotated behind a bronze grid. The grid consisted of 7 homocentric circles defining the month intervals and 24 curved rods defining the hours according to the “hour – month” diagram (“analemma”). The rotation of the disc was achieved through a pulley and a flexible chain with a counterweight and a weight-float which was lowered or lifted through the isochronous (=equal time) descent or ascent of the water level. This isochronous descent or ascent was ensured by the isochronous water outflow through a self-regulated controller of the constant level of the Ktesibios type.
Every day a pointer was placed successively on the corresponding one of the 365 holes of the disc periphery, which defined the days of the zodiac, and marked the 12 day and 12 night non-isochronous hours according to the season.