Empedocles' theory of respiration is remarkable for the use of an extended analogy with a clepsydra (water clock).
Water clocks, also known as clepsydrae, along with sundials, are possibly the oldest time-measuring instruments.
To provide a means of measuring time at night the Egyptians also invented the water-clock, or "clepsydra" as the Greeks later called it.
A water clock or clepsydra is any timekeeper operated by means of a regulated flow of liquid.
