Somebody wrote:

That's sounds alright and workin, but how does one find_out the exact horse power of a motor?

You set up a pony brake to measure foot lbs of absorbed power per unit
of time as well as the necessary electrical instrumentation to measure
input volts, amps and power factor.

You now crunch the data with your handy dandy slide rule, then plot
electrical data vs mechanical data.

I won't tell you how long ago we did this as a mechanical enginering lab
project, but the slide rule comment should give you a clue.

Lew