The unit horsepower came around when James Watt had his team of salesmen going around to different factories marketing the steam engine to them. The factories were often driven by various ways such as water wheels but ofted had horses to run the factory when the water did not flow enough. Horses would be tied to a windlass and pull this around all day to provide power for the axles that ran the factory. A factory owner would know how many horses they would have to keep in the stable in order to run the factory continuously. The salesmen would offer to replace the windlass, stables and horses with a single steam engine. They offered different sizes of engines depending on the size of the stable at the factory.
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