Where did the term “Boiler Horse Power” come from?

Joe SmolinskiRatings, Monday Morning Minutes, Boilers

Maybe you are all clear where the term Boiler HP came from, but I wasn’t, so I began my quest. I quickly found that one Mechanical HP = 33,000 ft-lb/min and one Boiler HP = 33, 475 BTUH and the similarity of these two number, without units, got my interest. It appears that James Watt needed a needed a marketing piece to sell his steam engines and people of the time