1 HP was meant to represent the average amount of power a horse could sustain for a work shift. A horse can’t sustain 15 HP for more than a few seconds at a time.
Peak horsepower is not that relevant to the sorts of applications that horses were performing that they proposed to use engines for. In that case they wanted sustained power, and a horse can only produce about 1 horsepower sustained.
Well, it's scientific in the same way that an accused witch and a duck look the same weight on the scale then. Besides, after a few minutes sous vide, I wouldn't be able to pull with all my usual strength, would you?
