Tugboat and barrel: what they have in common

1. Step 1. Tugboat — has part — Paddle wheel. The first tugboats were paddle-driven.
2. Step 2. Paddle wheel — works like — Water wheel. A water wheel in reverse: instead of water turning the wheel, the wheel pushes the water.
3. Step 3. Water wheel — uses — Gravity flow. Falling or flowing water turns the wheel.
4. Step 4. Gravity flow — is used in — Aqueduct. Water flows down a gradient without pumps.
5. Step 5. Aqueduct — has part — Arch. Roman aqueducts rested on arcades.
6. Step 6. 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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