Whistle edge and optical telegraph: what they have in common

1. Step 1. Whistle edge — is part of — Whistle. A sharp edge that splits the air stream.
2. Step 2. Whistle — uses — Sound wave. The sound of a flow.
3. Step 3. Sound wave — is used in — Phonograph. It records sound vibrations as a groove.
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.

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