Sometimes it is worthwhile to look back on where we have come from to assess where we are and where we are going. Items in my garage give us a clue.
First, there is my air compressor. Ceiling mounted, and connected to the distribution header I put in my home in order to have compressed air on each floor without having to drag around an air compressor, it is rated at 5 HP. Take the cheap plastic cover off the top of the unit and peer at the motor. You will find a smallish motor reminiscent of the one I built for a science fair about 60 years ago. However, if you look at the rpm at which it spins and do the calculations, you will indeed find its output to be 5 HP.
On the garage floor, less than fifteen feet away, you will observe a cast iron contraption, unfamiliar in this day. It is a cast iron engine, single cylinder, water cooled, with two flywheels, about 24 inches in diameter. It is an "Economy" brand name gasoline engine, approximately one hundred years old. Economy sold these under several brand names. This one likely came off a cement mixer. It produces 1 ½ HP, according to the nameplate, and weighs approximately 300 pounds.
Sitting near the Economy is a Maytag air-cooled one-cylinder engine (they also made two-cylinder ones). It has a kick starter, weighs about 15 pounds and produces about ¼ HP. These were sold integral to Maytag washing machines sold to rural areas where there was no electricity. Again, about 100 years ago. Conveniently, these came with a flexible steel exhaust pipe that the lady of the house could hang outside an open window in order to avoid asphyxiation.
Then, hanging from a support beam of the mezzanine in my garage is a David Bradley chain saw, roughly sixty years old. I bought this at an antique tractor show a dozen years or so ago. It is not the one, but it is identical to one my dad bought new around 1959 - 1960. It was state of the art at the time, had a 26-inch cutting bar and weighed 56 pounds. Don't know the horsepower rating. It cost about $250 in 1960 dollars!