Inflow water clock

In an outflow water clock, as the water level dropped, so did the water pressure and the rate of flow. The inflow water clock was developed to solve this problem. This device consists, in essence, of two vessels. The upper vessel is constantly filled with water faster than the water drains into the lower vessel. The excess water drains away, keeping the water level in the upper vessel -- and the rate at which it drains into the lower vessel -- constant. The time can therefore be measured by the level of the water in the lower vessel. 

An additional advantage of this arrangement is that a float in the lower vessel can be used to drive a geared mechanism which displays the time on a dial. Already at this stage in the tradition, the uniform circular motion of the heavens is represented by the uniform circular motion of the hand of the clock around the dial divided into 24 hours. 

Vitruvius (De architectura) attributes the invention of this kind of clock to Ctesibius of Alexandria (3rd cent. BC), who is also credited with inventing a hydraulic organ. A more complicated clock of this kind, attributed to Ctesibius, is show in the animated video.

Yet another line of development was to use the water power to create audible as well as visual effects.  An excellent example is an alarm clock in which rapidly inflowing water forces air from a container which makes a small bronze bird 'sing'.

Commentary. Howard Hotson (May 2021)