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Water Clock

Using the water clock

Choose how long the vessel should take to empty, press start, and watch the level fall. The digits give you the exact figure; the water gives you the feel of it, which is the whole point of a clock you can see rather than read.

Where water clocks come from

The clepsydra is one of the oldest timekeeping devices we can date. Examples survive from Egypt in the fourteenth century BC, and the idea spread to Babylon, Greece, China, India and Rome. For most of recorded history it was the only clock that worked at night or under cloud, which sundials cannot do.

Athenian courts used one to ration speeches — the phrase about a speaker’s water running out is literal. Roman legal practice did the same. In China, Su Song’s astronomical clock tower of 1092 used a water-driven escapement to turn an armillary sphere, a mechanism of real sophistication several centuries before the European weight-driven clock.

The hard engineering problem was that a full vessel drains faster than a nearly empty one, because pressure falls with the level. Greek and Islamic makers solved it either by keeping the supply vessel topped up so the head stayed constant, or by shaping the bowl so that the falling rate and the narrowing volume cancelled out.

What it is good for now

This one drains at a steady rate, because a screen has no pressure to worry about. That makes it a plain countdown with an unusually calm face — useful for meditation, breathing exercises, classroom transitions, a bath, or anything where a row of blinking digits would work against the mood.

If you want the same idea with a harder edge, the sand timer is more abrupt, and the countdown timer is the plain numeric version.

Questions people ask

What is a water clock?

A timekeeper that measures an interval by the steady flow of water out of, or into, a marked vessel. The Greek name is clepsydra, literally a water thief.

How old are they?

Egyptian examples survive from around the fourteenth century BC, and similar devices appear independently in Babylon, China, India and Mesoamerica.

Why were they important?

They were the only clocks that worked at night, indoors or under cloud, which is precisely when a sundial is useless.

Did courts really use them to time speeches?

Yes. Athenian and Roman courts allotted each speaker a vessel of water, and the phrase about a speaker’s time running out is a literal description.

What was the main engineering problem?

A full vessel drains faster than a nearly empty one, because pressure falls with the level. Makers solved it by keeping the supply topped up, or by shaping the bowl so the two effects cancel.

What was Su Song’s clock tower?

A water-driven astronomical clock built in China in 1092, with an escapement regulating a wheel that turned an armillary sphere — centuries ahead of comparable European mechanisms.

Does this one slow down as it empties?

No. It drains at a constant rate, because there is no real water and no pressure to compensate for.

What is it useful for today?

Anything where a calm visual cue beats blinking digits: meditation, breathing exercises, classroom transitions, a bath, a nap.

How long can I set it for?

From one minute to an hour, using the buttons under the vessel.

Will it alert me at the end?

Yes, with whichever of the fifty-one tones you have selected in the design panel.

What replaced water clocks?

Mechanical clocks with weight-driven escapements from the fourteenth century, and then the pendulum after Huygens in 1656, which was far more accurate.

Is a water clock the same as a water dial?

No. A water dial is a sundial marked for reading; a water clock measures elapsed time by flow and needs no sun at all.

Where to go next

Bookmark or share water clock directly. Everything else is one step away on world time.