Instant messenger e railway semaphore: o que têm em comum

1. Passo 1. Instant messenger — evoluiu de — SMS. Messengers displaced SMS.
2. Passo 2. SMS — funciona como — Electrical telegraph. A short text message is the heir of the telegram.
3. Passo 3. Electrical telegraph — evoluiu de — Optical telegraph. The electrical telegraph replaced semaphore lines.
4. Passo 4. Optical telegraph — evoluiu para — Railway semaphore. The semaphore idea.

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