How Horsepower is equal to 550 ft lb/s ?

[PHYSICS5502]

How did James Watt found out that one horsepower is equal to 550 ft lb/s ? There must be some sort of derivation or something that he did.Can anyone show me that derivation.

 

[Nidum]

From observation of the haulage power of a horse . He fiddled the answer a lot though for commercial purposes .

Boulton and Watt manufactured steam engines .They were a bit notorious for making exaggerated claims about the power and economy of their engines compared to horses .

https://en.wikipedia.org/wiki/Horsepower#History_of_the_unit