Smelting and conveyor belt: what they have in common

1. Step 1. Smelting — uses — Charcoal. Before coke, metal was smelted with charcoal.
2. Step 2. Charcoal — is material for — Gunpowder. Gunpowder is a mix of saltpetre, sulphur and charcoal.
3. Step 3. Gunpowder — evolved into — Dynamite. A replacement for gunpowder in mining.
4. Step 4. Dynamite — works like — Tunnelling shield. Two ways of driving tunnels — mechanical and explosive.
5. Step 5. Tunnelling shield — evolved into — Tunnel boring machine. A development of the tunnelling shield.
6. Step 6. Tunnel boring machine — has part — Conveyor belt. Spoil is removed by a conveyor.

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