Technology and Engineering · Middle school

Inside the Elephant Clock

How does the sinking bowl in al-Jazari’s Elephant Clock measure half an hour? Scrub through time and follow it.

In the learning section below the figure, predict, try, complete the tasks and check yourself with short questions.

Fig. 1Inside the Elephant Clock

Drag the time, press Play or take the clock apart

Parts and the hidden mechanism

Apart0%

With the clock apart, hover over a part, tap it or pick it from the list; use the arrow keys to move through the list.

Pick an assembly to see what it does and what it is made of.

Time elapsed

Bowl fill time

Hole diameter: 1.00× the hole in the article

Time elapsed: 20.0 min
Bowl filled: 68%
Balls dropped: 0 / 8
Ball interval: 30 min

Water seeps in through the hole in the bowl’s bottom; as the bowl fills it sinks by the same amount.

Learn

Progress is kept on this page only; no points are given.

1Predict
2Try
3Explain

If you make the hole in the bowl’s bottom half as wide, how long does the bowl take to fill?

Gold: the balls, and the roundels that count the dropped ballsPurple: the tank water and the water filling the bowlThe cycle follows the order given in the article. The bowl is taken as a thin-walled cylinder: as the water inside rises, the bowl sinks by the same amount, so the pressure difference across the hole stays the same and the bowl fills at a steady rate. The flow through the hole then depends only on its area: when you change the fill time, the hole diameter is worked out as inversely proportional to the square root of that time, and the ball interval is the fill time plus the half minute of sinking and ball drop. Simplifications: the number of balls in the tower (8), how the strings run over pulleys, the seesaw gate, the bowl draining once lifted, and the pointer and roundels of the dial are illustrative; the article does not describe the original dial or sound mechanism in detail, and since accounts differ on whether the sound came from a cymbal or a drum, a drum is drawn here. During playback, time runs 177 times faster while the bowl fills (filling takes about 10 seconds) and 2.5 times faster during the sinking and the ball drop. The positions in the taken-apart view are only for display; the materials and decoration are a reconstruction, and the hole is enlarged in the cutaway so it can be seen.

Al-Jazari and the Elephant Clock: engineering in the Middle AgesRead the article