District heating and tunnelling shield: what they have in common

1. Step 1. District heating — is used in — Central heating. Connection to the heating network.
2. Step 2. Central heating — evolved from — Hypocaust. The Roman hypocaust was an early central heating system.
3. Step 3. Hypocaust — is made of — Brick. The pillars of a hypocaust are brick.
4. Step 4. Brick — is material for — Metro. The vaults of the first tunnels are brick.
5. Step 5. Metro — uses — Tunnelling shield. Metro tunnels were built with shields.

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