Public address system and telescope: what they have in common

1. Step 1. Public address system — has part — Loudspeaker. Loudspeakers.
2. Step 2. Loudspeaker — is part of — Telephone. The earpiece reproduces the other person’s voice.
3. Step 3. Telephone — evolved from — Electrical telegraph. The telephone grew out of attempts to send several signals over telegraph lines (the “harmonic telegraph”).
4. Step 4. Electrical telegraph — evolved from — Optical telegraph. The electrical telegraph replaced semaphore lines.
5. Step 5. Optical telegraph — uses — Telescope. Signals were read through a telescope.

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