Reaction wheel and railway semaphore: what they have in common

1. Step 1. Reaction wheel — is part of — Space telescope. Pointing Hubble.
2. Step 2. Space telescope — is a kind of — Telescope. A space telescope is a telescope above the atmosphere, so air does not blur the view.
3. Step 3. Telescope — is used in — Optical telegraph. Signals were read through a telescope.
4. Step 4. Optical telegraph — evolved into — Railway semaphore. The semaphore idea.

Open Evothings