Zhang Heng’s seismoscope and saw: what they have in common

1. Step 1. Zhang Heng’s seismoscope — uses — Pendulum. An internal pendulum (according to reconstructions).
2. Step 2. Pendulum — uses — Universal gravitation. The restoring force is gravity.
3. Step 3. Universal gravitation — is used in — Gravity flow. It flows downhill under gravity.
4. Step 4. Gravity flow — is used in — Water wheel. Falling or flowing water turns the wheel.
5. Step 5. Water wheel — is part of — Sawmill. Early sawmills were driven by water wheels.
6. Step 6. Sawmill — uses — Saw. The cutting tool is a saw.

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