Thermodynamics and railway semaphore: what they have in common

1. Step 1. Thermodynamics — was inspired by — Watt steam engine. Carnot wrote “Reflections on the Motive Power of Fire” trying to understand steam engines.
2. Step 2. Watt steam engine — uses — Coal. Steam engines were fired with coal.
3. Step 3. Coal — is material for — Coal gas. The gas was made by the dry distillation of coal.
4. Step 4. Coal gas — is used in — Traffic light. A gas lamp lit the signal.
5. Step 5. Traffic light — evolved from — Railway semaphore. The first traffic light was a railway-style semaphore.

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