Water clock
A sundial falls silent at night and on cloudy days. Water always flows. For three thousand years people tried to make it flow evenly, and in Alexandria they very nearly managed it.
- c. 1380 BCthe oldest surviving water clock, Karnak
- c. 270 BCCtesibius's water clock
- 70 mina summer hour in Alexandria; a winter one was 51
A pot with a hole
The Greek word klepsydra means "water thief". In the Athenian courts such a pot stood before the speaker: he could talk while the water ran out, and when it was empty his speech was over.
The oldest surviving water clock is an alabaster vessel from the temple at Karnak, from the reign of Pharaoh Amenhotep III. Rows of marks are cut inside, and the priests read the hour of the night from the water level.
Fast at first, then slow
The higher the column of water, the harder it presses on the hole and the faster the jet. In a full cylinder the level races down at first and crawls at the end. Torricelli only worked out the law in 1643: the jet's speed grows with the square root of the height.
The blue line on the chart is the cylinder. With an evenly spaced scale, such a clock would be out by hours in a single night.
Sloping walls
The Egyptian makers didn't know the formula, but they knew the result. The walls of the Karnak vessel flare outward at about 70° to the horizontal. The vessel is wider at the top, so the same jet lowers the level more slowly just when the jet is strongest.
Move the slider under the chart and find the slope at which the line is nearly straight.
Table of values
A barber's son from Alexandria
Around 270 BC the Alexandrian engineer Ctesibius turned the problem around. Water runs out unevenly because its level falls. So the level must not fall at all.
His cistern is filled from above, and any surplus leaves by an overflow pipe. A float valve closes the supply when the water reaches the mark. The level in the cistern stays put, so the pressure and the jet never change.
A hole that doesn't wear
Water drips from the cistern through a tiny hole. Vitruvius writes that it was bored in gold or in a gemstone: copper corrodes and clogs over time, and the clock starts to drift.
Water lifts the pointer
The drops fall into a cylinder, and a float rises in it. On the float's rod stands a small figure pointing at a column of hour lines. Since the water comes in evenly, the figure rises evenly too.
A column that turns
Greeks and Romans divided the daylight into twelve hours, so a summer hour was longer than a winter one. In Alexandria a summer hour lasted about 70 minutes.
Ctesibius drew the hour lines on a drum and turned it through the year. Move the pointer or swipe sideways over the drawing to change the season: the lines on the column spread apart or close up.
A short winter day
In December daylight in Alexandria lasts about ten hours, and each of the twelve hours shrinks to 51 minutes. By sunset the float never reaches the top of the column.
What happens without a constant level
We have removed the float valve. The cistern no longer refills, its level falls, the jet weakens, and the figure falls further and further behind the Sun. The readout in the drawing's corner shows by how many minutes.
Water after Alexandria
Water clocks lived on for another fifteen hundred years. In 1088 Su Song built a tower at Kaifeng in which water turned a wheel of buckets. In 1206 al-Jazari described the elephant clock: every half hour a bowl sank inside it and the mechanism struck the time.
Water had two enemies: it froze in winter, and over time it clogged the hole. When clocks with a weight and a verge escapement appeared around 1300, water was no longer needed.