The water clock shown here does not have all of Ktesibios’s features, but it provides an illustration regarding how these devices operate. This is from The Cyclopaedia or Universal Dictionary of Arts, Sciences and Literature by Abraham Rees in 1820.

c. 250 BCE

KTESIBIOS’S WATER CLOCK

“Ktesibios’s water clock was significant because it forever changed our understanding of what a man-made object could do,” writes journalist Luke Dormehl. “Before Ktesibios’s clock, only a living thing was thought to be capable of modifying its behavior according to changes in the environment. After Ktesibios’s clock, self-regulating feedback control systems became a part of our technology.”

The Greek inventor Ktesibios, or Tesibius (fl. 285–222 BCE), was famous in Alexandria, Egypt, for his devices involving pumps and hydraulics. His water clock, or clepsydra (lit. “water thief”), is of particular interest because it employed a regulator in the form of a feedback-control float that maintained a constant water-flow rate, thus allowing his timepiece to provide reasonable estimates of time according to the level of water in a receiving container. In one version of his clock, units of time are marked on a column that a humanoid figure points to as he rises with the changing water level in the reservoir. According to some reports, the humanoid figure was accompanied by other mechanisms, such as turning pillars and falling stones or eggs, along with trumpet-like sounds. Ktesibios’s clepsydras were used to allocate time to speakers in court proceedings and to limit time spent by patrons in Athenian brothels.

Ktesibios was likely the first head of the Museum of Alexandria, an institution that included the Library of Alexandria and that attracted leading scholars of the Hellenistic world. Although he is famous for his particular kinds of clepsydras, other related water clocks were also built in ancient China, India, Babylon, Egypt, Persia, and elsewhere. Ktesibios was also reported to have invented an eerie robotic statue of a deity that was featured in processions (e.g., the famous Grand Procession parade of Ptolemy Philadelphus). This automaton was able to stand up and sit down via the rotation of cams (non-circular wheels that convert circular motion to linear motion) that were perhaps linked to the movement of a cart.