Barrel and sonar: what they have in common

1. Step 1. Barrel — works like — Arch. Staves squeezed by hoops hold their shape by the same compression as the stones of an arch.
2. Step 2. Arch — evolved into — Dome. A dome is an arch rotated around an axis.
3. Step 3. Dome — is part of — Helmet. A protective shell.
4. Step 4. Helmet — is made of — Leather. Helmet liners and simple helmets were made of leather.
5. Step 5. Leather — is material for — Submarine. It was covered with leather.
6. Step 6. Submarine — has part — Sonar. Underwater acoustics.

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