How Do You Measure Time With Water? Flow That Refuses to Be Steady

The simplest form is a vessel with a small hole, filled with water and marked on the inside, where the falling level indicates elapsed time. It works and it is not accurate, because the rate at which water leaves depends on the pressure behind it, which depends on the depth remaining, so a full vessel drains faster than a nearly empty one and the marks cannot be evenly spaced. Two responses are available. The marks can be spaced unevenly to match the actual behaviour, which requires calibration and works only for that vessel. Or the design can be changed so the pressure stays constant, which is the route that led to genuinely useful instruments and to a great deal of ingenuity.

Several arrangements keep the driving pressure constant:

• •An overflow reservoir feeding the measuring vessel, kept full so the head never changes
• •A float valve admitting water at exactly the rate it leaves
• •A shaped vessel whose cross-section compensates for the falling rate
• •Inflow devices, where water accumulates in a vessel and the rising level is read
• •A float carrying a pointer or driving a mechanism as the level changes
• •Escapement arrangements tipping a filled container, which converts flow into discrete events

The applications explain why so much effort went into them. Ancient Greek and Roman courts used them to time speeches, with a fixed allocation of water per speaker and with the clock stopped while documents were read, which is where the expression about somebody's time running out comes from. Astronomical observation required timing that the sun could not supply, particularly at night, and water clocks were the instrument. Religious observance requiring prayer at set hours needed something working after dark. Mines, irrigation systems and public baths allocated resources by time. And rulers commissioned elaborate examples as displays of technical capability, with several described in detail in surviving texts and at least one sent as a diplomatic gift between courts.

What a clock was expected to show constrained its design in ways that are easy to miss. Where hours were defined as twelfths of the daylight period, their length changed through the year, so a device dividing a fixed interval had to be adjustable or recalibrated seasonally, which several designs handled with interchangeable scales or with a drum rotated to a different marked band as the seasons changed. That requirement disappears only once hours are defined as fixed fractions of a day and night together, which happened gradually and unevenly. Night hours also needed measuring, and since sundials fail entirely after dark, the water clock was the only continuous option, which is why astronomical and religious use drove the technology more than ordinary civil timekeeping did.

The most developed examples were substantial machines. Chinese astronomical clocks built from the eighth century onward drove rotating celestial globes and figures that struck bells, with one built in the eleventh century standing some ten metres high and incorporating an escapement mechanism that released a driving wheel at controlled intervals, which is a genuine ancestor of mechanical clock escapements. Devices described in the Islamic world included figures that moved, doors that opened and musical automata driven by water. Water clocks remained the most accurate timekeepers available until the pendulum was applied to mechanical clocks in the seventeenth century, which improved accuracy by roughly two orders of magnitude and ended the tradition within a generation.

Water drains faster from a full vessel than an empty one, so marks cannot be evenly spaced unless the driving pressure is held constant, which is what an overflow reservoir or a float valve achieves. They timed court speeches, astronomical observation and prayer. Chinese astronomical examples incorporated an escapement releasing a wheel at intervals, which anticipates mechanical clocks.