Leibniz’s stepped reckoner and tunnelling shield: what they have in common

1. Step 1. Leibniz’s stepped reckoner — evolved into — Arithmometer. Thomas’s arithmometer used Leibniz’s stepped drum.
2. Step 2. Arithmometer — has part — Crank. The handle for adding and subtracting.
3. Step 3. Crank — is part of — Steam locomotive. Connecting rods and cranks carry the piston stroke to the wheels.
4. Step 4. Steam locomotive — uses — Railway track. It runs on rails.
5. Step 5. Railway track — is used in — Metro. It runs on rails.
6. Step 6. Metro — uses — Tunnelling shield. Metro tunnels were built with shields.

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