Stopwatch and railway semaphore: what they have in common

1. Step 1. Stopwatch — has part — Mainspring. A mainspring.
2. Step 2. Mainspring — is part of — Gramophone. A spring drive.
3. Step 3. Gramophone — evolved from — Phonograph. A flat disc instead of a cylinder.
4. Step 4. Phonograph — was inspired by — Electrical telegraph. Edison came to the phonograph while working on recording telegraph and telephone signals.
5. Step 5. Electrical telegraph — evolved from — Optical telegraph. The electrical telegraph replaced semaphore lines.
6. Step 6. Optical telegraph — evolved into — Railway semaphore. The semaphore idea.

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