Whistle and optical telegraph: what they have in common

1. Step 1. Whistle — uses — Sound wave. The sound of a flow.
2. Step 2. Sound wave — is used in — Phonograph. It records sound vibrations as a groove.
3. Step 3. Phonograph — was inspired by — Electrical telegraph. Edison came to the phonograph while working on recording telegraph and telephone signals.
4. Step 4. Electrical telegraph — evolved from — Optical telegraph. The electrical telegraph replaced semaphore lines.

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