Babbage’s Difference Engine and optical telegraph: what they have in common

1. Step 1. Babbage’s Difference Engine — evolved into — Babbage’s Analytical Engine. Babbage moved on to a general-purpose machine while working on the difference engine.
2. Step 2. Babbage’s Analytical Engine — inspired — Harvard Mark I. Howard Aiken named Babbage’s work as the source of his idea.
3. Step 3. Harvard Mark I — has part — Relay. The Mark I was electromechanical, built from relays.
4. Step 4. Relay — is part of — Electrical telegraph. Relays boosted the signal on long lines.
5. Step 5. Electrical telegraph — evolved from — Optical telegraph. The electrical telegraph replaced semaphore lines.

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