Stopwatch e railway semaphore: o que têm em comum

1. Passo 1. Stopwatch — tem como parte — Mainspring. A mainspring.
2. Passo 2. Mainspring — faz parte de — Gramophone. A spring drive.
3. Passo 3. Gramophone — evoluiu de — Phonograph. A flat disc instead of a cylinder.
4. Passo 4. Phonograph — foi inspirado por — Electrical telegraph. Edison came to the phonograph while working on recording telegraph and telephone signals.
5. Passo 5. Electrical telegraph — evoluiu de — Optical telegraph. The electrical telegraph replaced semaphore lines.
6. Passo 6. Optical telegraph — evoluiu para — Railway semaphore. The semaphore idea.

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