Can water make a machine follow the sky?

That time, that place

Kaifeng, Northern Song. Observing the heavens and fixing the calendar were affairs of state: if the emperor ruled by the mandate of heaven, he had to be seen to know that heaven precisely.

Why this question

Instruments posed one nagging problem: to follow the sky you must keep turning them, and a hand cannot turn anything evenly. The heavens move at a constant rate; hands do not.

Why not a waterwheel? Because water simply pouring in makes the wheel accelerate. Now it is uneven in the other direction.

What was found

What Su Song built was a twelve-meter tower, and at its heart sat one device. When a bucket filled to a set amount, a catch released, the wheel advanced exactly one notch, and the catch caught it again. Fill, one notch. Fill, one notch.

What this did was cut a continuous flow into a steady beat — the principle at the heart of every mechanical clock.

Driven by it, an armillary sphere at the top tracked the sky, while on the floors below figures emerged to announce the hours. A device on the same principle appears in Europe two centuries later.