Pressure chamber and tunnelling shield: what they have in common

1. Step 1. Pressure chamber — is part of — Autoclave. A sealed chamber.
2. Step 2. Autoclave — has part — Boiler. A steam generator.
3. Step 3. Boiler — is part of — Excavator. The first excavators were steam-powered.
4. Step 4. Excavator — has part — Hydraulic cylinder. Hydraulic cylinders move the boom and the bucket.
5. Step 5. Hydraulic cylinder — is used in — Tunnelling shield. The shield was pushed forward by jacks.

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