This section is from the "The New Student's Reference Work Volume 5: How And Why Stories" by Elinor Atkinson.
When James Watt invented the stationary steam engine, one of the difficulties he had was to be certain how large an engine was needed to do a given amount of work. He would get an order something like this: "An engine is wanted to pump water from a mine. It must do the work that is now done by twenty-four horses." The thing he had to find out was just how much work one horse can do. He tested many horses until he found that the average force of the working horses then used, was just enough to raise thirty-three thousand pounds one foot high in one minute of time. Allowance also had to be made for friction, or rubbing of the parts of the machinery, and other things. Today horse-power in an engine is not calculated in that way. It refers to the size of the cylinder. A horse power now may mean a lift of sixty thousand pounds one foot high in one minute.