Bronze dragon heads holding balls and sewage treatment plant: what they have in common

1. Step 1. Bronze dragon heads holding balls — is part of — Zhang Heng’s seismoscope. Eight directions.
2. Step 2. Zhang Heng’s seismoscope — uses — Pendulum. An internal pendulum (according to reconstructions).
3. Step 3. Pendulum — is used in — Tuned mass damper. A pendulum.
4. Step 4. Tuned mass damper — is made of — Concrete. Or concrete.
5. Step 5. Concrete — is material for — Sewage treatment plant. The tanks are concrete.

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