Keel and oil lamp: what they have in common

1. Step 1. Keel — is part of — Sailing ship. A keel.
2. Step 2. Sailing ship — has part — Sextant. The sextant and chronometer were a sailing ship’s navigation kit for finding latitude and longitude.
3. Step 3. Sextant — has part — Lens. The sextant’s telescope.
4. Step 4. Lens — is part of — Magic lantern. Lenses.
5. Step 5. Magic lantern — uses — Oil lamp. The light source is an oil lamp.

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