A man who selled beer has bought the steam engine from English scientist and inventor George Watt (1736-1819) The man asked him a question How many horses the steam can replace ?  Watt answered  It replaces 3 or 4 horses . But the man wanted to make sure about that.So they chose a horse and let it work 8 hours without stopping.They could define the horse  power. Since then appeared a term  Horse Power . Horse power : When a horse lifts 75 kgs weight on 1 meter  altitude refers to as one horse power.