Shadow vane and submarine: what they have in common

1. Step 1. Shadow vane — is part of — Davis quadrant. It casts the Sun’s shadow onto the slit.
2. Step 2. Davis quadrant — evolved into — Octant. Reflecting angle gauges.
3. Step 3. Octant — has part — Mirror. Two mirrors.
4. Step 4. Mirror — is part of — Periscope. Mirrors or prisms.
5. Step 5. Periscope — is part of — Submarine. A periscope.

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