Boomerang and electrical telegraph: what they have in common

1. Step 1. Boomerang — uses — Angular momentum. Gyroscopic precession brings the boomerang back.
2. Step 2. Angular momentum — is used in — Reaction wheel. Conservation of angular momentum.
3. Step 3. Reaction wheel — is part of — Space telescope. Pointing Hubble.
4. Step 4. Space telescope — is a kind of — Telescope. A space telescope is a telescope above the atmosphere, so air does not blur the view.
5. Step 5. Telescope — is used in — Optical telegraph. Signals were read through a telescope.
6. Step 6. Optical telegraph — evolved into — Electrical telegraph. The electrical telegraph replaced semaphore lines.

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