Water clock
From chronoskin, the open reference on timekeeping
For the sinking-bowl timer used to share out irrigation turns, see Water clock (irrigation).
A water clock is an instrument that measures time by the regulated flow of liquid into or out of a vessel, where the amount is then read against a scale. With the sundial it is among the oldest instruments for telling the hour, and unlike the sundial it works indoors, under cloud and at night.[1]
The simplest form is a bowl with a small hole near its base. Makers soon found that the flow slows as the level falls, and much of the later history of the instrument is the search for an even rate.[2]
Contents [hide]
[edit]Principle
[edit]Outflow clocks
An outflow clock is a vessel that drains through a narrow opening. Hours are marked on its inner wall, and the wall is often sloped so that equal drops in level stand for nearly equal times. A formula of the form t = k * sqrt(h) describes the emptying of a straight-sided vessel.
[edit]Inflow clocks
An inflow clock collects the water in a second, straight-sided vessel in which a float carries a pointer. Because the receiving vessel does not affect the rate, the scale can be evenly divided, provided the supply is steady. The usual remedies were:
- an overflow tank that keeps the head of water constant;
- a conical float valve that closes the inlet as the level rises;
- a set of interchangeable scales, one for each month, where hours varied with the season.
[edit]Accuracy
This section needs more sources. Figures below come from a single modern reconstruction. Help improve it by adding measurements from published tests.
Temperature changes the thickness of water and so the rate of flow; a clock set in summer runs slow in winter. Tests on modern replicas give the figures below.[3]
| Form | Regulator | Run time (h) | Error per day (min) |
|---|---|---|---|
| Plain outflow bowl | None | 6 | 45 |
| Sloped outflow vessel | Wall profile | 12 | 20 |
| Inflow with overflow tank | Constant head | 24 | 12 |
| Inflow with float valve | Feedback valve | 24 | 10 |
[edit]See also
- Sundial (a link to an article)
- Sandglass (an article you have already read)
- Candle clock (under the pointer)
- Pendulum clock (while pressed)
- Incense clock (a page nobody has written yet)
- Museum catalogue of flow instruments (an outside link)
[edit]References
- ^ Aldous, M. Instruments of the Hour, 2nd ed., pp. 14 to 31.
- ^ Brandt, K. "Flow and level in draining vessels", Journal of Early Mechanics 12, pp. 201 to 219.
- ^ Field notes of the replica trials, open workshop archive. Retrieved on 4 March 2007.
Categories: Timekeeping instruments | Clocks | Ancient inventions | Hydraulic devices