Trunnions and rock drill: what they have in common

1. Step 1. Trunnions — is part of — Bessemer converter. They tilt the converter to pour out the steel.
2. Step 2. Bessemer converter — evolved into — Open-hearth furnace. A more controllable process.
3. Step 3. Open-hearth furnace — is made of — Brick. An open-hearth furnace is built of firebrick.
4. Step 4. Brick — is material for — Metro. The vaults of the first tunnels are brick.
5. Step 5. Metro — uses — Tunnel. Tunnels.
6. Step 6. Tunnel — uses — Rock drill. Driving the Mont Cenis tunnel.

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