Vent windows and tunnelling shield: what they have in common

1. Step 1. Vent windows — is part of — Greenhouse. Venting when overheated.
2. Step 2. Greenhouse — inspired — Crystal Palace. Paxton was a gardener and a builder of glasshouses.
3. Step 3. Crystal Palace — is made of — Cast iron. Cast iron.
4. Step 4. Cast iron — is material for — Tunnelling shield. Brunel’s shield is a cast-iron frame.

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