To be honest, I’m not really sure what “horsepower” refers to in cars. I have a vague idea it’s got something to do with speed or power, but I have no idea beyond that.

And now I think about it, what about horses? Surely a horse should have one horsepower (HP), right?

Well, according to a 1993 letter written between two scientists, that’s completely wrong.

Per Britannica’s online encyclopedia, “horsepower” is a unit of power invented by the Scottish engineer James Watt in the late 1700s.

In British Imperial measures, they say: “One horsepower equals 33,000 foot-pounds of work per minute — that is, the power necessary to lift a total mass of 33,000 pounds one foot in one minute.”

The term is actually related to horses, too. James Watt conducted a series of experiments on strong horses to see how much power they exerted over a normal day’s work.

Horsepower can also be expressed as watts (746 watts) and heat (92,545 BTU, or British Thermal Units, per hour).

For engines in cars, planes and other vehicles, “indicated horsepower” can refer to the pressure in the cylinders.

Then, there’s “brake” or “shaft” horsepower, which means “less than indicated horsepower by the amount of power lost to friction within the engine itself,” Britannica reads.

So if you’ve always wondered whether horsepower refers to force, pressure, heat, or watts, the answer is “yes.”

In 1993, biologists R. D. Stevenson and R. J. Wassersug sent a letter to the science journal Nature.

In it, they tried to work out how much horsepower a horse could really have.

They calculated that in theory, a horse at its absolute peak performance may be able to achieve 24 horsepower.

But after looking at real-life data from a 1925 Iowa State Fair, they found the real-world value was far lower ― in a short time span, they found, horses can get to around 1.49 horsepower.