Can a clock announce the hour with nobody watching it?

That time, that place

The water clock is ancient. Let water flow at a steady rate and read the hour from how high it has risen.

The trouble was the reader. Someone had to watch the scale through the night and strike a bell at the appointed hour. That bell opened and closed the city gates and set the curfew, so early or late meant the country's day was out of joint.

Mistakes were punished. The design placed a human being between sleepiness and penalty.

Why this question

What Jang Yeong-sil built was not a more accurate water clock. It was a water clock with the person taken out.

As water rises, a rod floats up inside the vessel. Rising, it trips a catch and a small iron ball rolls free. The ball runs along a fixed channel, trips the next catch, and that releases a larger ball.

At the end the force lifts a lever, and wooden figures swing their arms to strike a bell, a drum, a gong. A different figure appears for each of the twelve divisions of the day.

The heart of it is the conversion: a continuous rise in water level turned into a discrete event, the moment a ball drops.

What was found

From 1434 the self-striking clepsydra was Joseon's standard clock. It stood in a pavilion in Gyeongbok Palace, and the hours it gave were relayed to the bell tower and out across the capital.

This was not the making of a clock but the setting of a single national time standard — the same kind of act as the rain gauge that made rainfall comparable across the country. Sejong-era projects generally have this shape: the device is not the point, the system built around it is.

Jang Yeong-sil was born a government slave. His skill was noticed, he was raised into office, and he reached the third senior rank. Then in 1442 a royal palanquin he had overseen broke, and after that his name disappears from the record. Where or when he died is unknown.