Zhang Heng · Seismoscope
Can you detect a distant earthquake without leaving the room?
That time, that place
Luoyang, capital of the Later Han. Zhang Heng held office over astronomy and the calendar. Earthquakes were a frequent disaster, and word of a distant one could take weeks to reach the capital. To send relief, you first had to know where the ground had given way.
Why this question
There is only one way to know faster than a rider can travel: catch the shaking itself. Even a tremor too faint for anyone in the capital to feel — might a machine feel it?
What was found
He built a great bronze vessel. Around it, eight dragons faced eight directions, each holding a ball in its mouth, and beneath each dragon sat a toad with its mouth open.
When the ground shook, a weight inside shifted and the dragon on that side released its ball. The sound of the ball dropping into the toad's mouth brought someone running, and whichever dragon had emptied showed the direction the shaking had come from.
In the year 138 no one in Luoyang felt anything, yet a ball fell. Days later, word arrived of an earthquake hundreds of li to the west.
An earthquake was known only when news of it arrived
The account preserved in the Book of the Later Han
Its inner mechanism was never recorded, and rival reconstructions still compete today