Signal wire and radar: what they have in common

1. Step 1. Signal wire — is part of — Railway semaphore. A cable from the control post.
2. Step 2. Railway semaphore — evolved from — Optical telegraph. The semaphore idea.
3. Step 3. Optical telegraph — evolved into — Electrical telegraph. The electrical telegraph replaced semaphore lines.
4. Step 4. Electrical telegraph — is used in — Weather forecast. Collecting reports by telegraph.
5. Step 5. Weather forecast — uses — Doppler radar. Weather radars.
6. Step 6. Doppler radar — is a kind of — Radar. A radar.

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