Silica (sand) and barrel: what they have in common

1. Step 1. Silica (sand) — is material for — Water filter. Slow filters are layers of sand and gravel.
2. Step 2. Water filter — uses — Gravity flow. Water seeps through by gravity.
3. Step 3. Gravity flow — is used in — Aqueduct. Water flows down a gradient without pumps.
4. Step 4. Aqueduct — has part — Arch. Roman aqueducts rested on arcades.
5. Step 5. Arch — works like — Barrel. Staves squeezed by hoops hold their shape by the same compression as the stones of an arch.

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