Compass legs and cannon: what they have in common

1. Step 1. Compass legs — is part of — Compasses. A needle and a lead holder.
2. Step 2. Compasses — is used in — Mercator projection. The map grid was built with compasses and a ruler.
3. Step 3. Mercator projection — is used in — Sailing ship. Navigating along straight rhumb lines.
4. Step 4. Sailing ship — includes — Galleon. A sailing ship.
5. Step 5. Galleon — has part — Cannon. Cannons on the decks.

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