Odometer and barrel: what they have in common

1. Step 1. Odometer — has part — Gear. Gear counters.
2. Step 2. Gear — is part of — Tower clock. A gear train drives the hands.
3. Step 3. Tower clock — is part of — Gothic cathedral. A tower clock.
4. Step 4. Gothic cathedral — has part — Vault. Rib vaults.
5. Step 5. Vault — evolved from — Arch. An extended arch.
6. Step 6. Arch — works like — Barrel. Staves squeezed by hoops hold their shape by the same compression as the stones of an arch.

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