Today, it has become common knowledge that the term "horsepower" refers to the power of an engine. We have come to assume that a car with a 400-horsepower engine will go faster than a car with a 130-horsepower engine. But with all due respect to the noble steed, some animals are stronger.
Why, for example, don't we brag about our engine's "oxenpower" or "bullpower" today? Scottish engineer James Watt knew he had a good thing going for him in the late 1760s when he came up with a greatly improved version of the first commercially available steam engine Thomas Newcomen had designed in 1712.
By adding a separate condenser, Watt's design eliminated the constant coal-wasting cycles of cooling and re-heating required by Newcomen's steam engine. Besides being an accomplished inventor, Watt was also a dedicated realist. He knew that in order to prosper from his ingenuity, he had to actually sell his new steam engine - to lots of people. So, Watt went back to work, this time to "invent" a simple way to explain the power of his improved steam engine in a way that his potential customers could easily understand.
Knowing that most people who owned Newcomen's steam engines used them for tasks involving pulling, pushing, or lifting heavy objects, Watt recalled a passage from an early book in which the author had calculated the potential energy output of mechanical "engines" that could be used to replace horses for such jobs. In his 1702 book The Miner's Friend, English inventor and engineer Thomas Savery has written:
"So that an engine which will raise as much water as two horses, working together at one time in such a work, can do, and for which there must be constantly kept ten or twelve horses for doing the same. Then I say, such an engine may be made large enough to do the work required in employing eight, ten, fifteen, or twenty horses to be constantly maintained and kept for doing such a work…"
After doing some very rough calculations, Watt decided to claim that just one of his improved steam engines could produce enough power to replace 10 of cart-pulling horses - or 10 "horsepower." Voila! As Watt's steam engine business soared, his competitors started advertising the power of their engines in "horsepower," thus making the term a standard measure of engine power still used today.
By 1804, Watt's steam engine had replaced the Newcomen engine, leading directly to the invention of the first steam-driven locomotive. In rating his steam engines at "10 horsepower," Watt had made a slight error. He had based his math on the power of Shetland or "pit" ponies that, because of their diminutive size, were typically used to pull carts through the shafts of coal mines.
A well-known calculation at the time, one pit pony could haul one cart filled with 220lb of coal 100 feet up a mineshaft in 1 minute, or 22,000 lb-ft per minute. Watt then incorrectly assumed that regular horses must be at least 50% stronger than pit ponies, thus making one horsepower equal to 33,000 lb-ft per minute. In fact, a standard horse is only slightly more powerful than a pit pony or equal to about 0.7 horsepower as measured today.
In a Famous Race of Horse vs. Steam, Horse Wins
In the early days of American railroading, steam locomotives, like those based on Watt's steam engine, were considered too dangerous, weak, and unreliable to be trusted with transporting human passengers. Finally, in 1827, the Baltimore and Ohio Railroad company, the B&O, was granted the first U.S. charter to transport both freight and passengers using steam-driven locomotives.
Despite having the charter, the B&O struggled to find a steam engine capable of traveling over steep hills and rough terrain, forcing the company to rely mainly on horse-drawn trains. To the rescue came industrialist Peter Cooper who offered to design and build, at no charge to the B&O, a steam locomotive he claimed would render horse-drawn railcars obsolete. Cooper's creation, the famed "Tom Thumb" became the first American-built steam locomotive running on a commercially-operated, public railroad.
Of course, there was a motive behind Cooper's apparent generosity. He just happened to own acre-upon-acre of land located along the B&O's proposed routes, the value of which would grow exponentially should the railroad, powered by his Tom Thumb steam locomotives, succeed.
On August 28, 1830, Cooper's Tom Thumb was undergoing performance testing on the B&O tracks outside of Baltimore, Maryland, when a horse-drawn train stopped alongside on the adjacent tracks. Casting the steam-powered machine a disrespectful glance, the driver of the horse-drawn train challenged the Tom Thumb to a race. Seeing winning such an event as a great, and free, advertising showcase for his engine, Cooper eagerly accepted and the race was on.
The Tom Thump quickly steamed to a large and growing lead, but when one of its drive belts broke, bringing the steam locomotive to a stop, the old reliable horse-drawn train won the race. While he had lost the battle, Cooper won the war. Executives of the B&O had been so impressed by his engine's speed and power that they decided to begin using his steam locomotives on all of their trains.
The B&O grew to become one of the largest and most financially successful railways in the United States. Profiting handsomely from sales of his steam engines and land to the railroad, Peter Cooper enjoyed a long career as an investor and philanthropist. In 1859, money donated by Cooper was used to open the Cooper Union for the Advancement of Science and Art in New York City.
Robert Longley has served as About.com/ThoughtCo's Expert on U.S. Government since October 1997.