Why do we still measure things in horsepower?

If you're buying a car and have no experience with power measurements or vehicle stats, you may be baffled by one of the vehicle's key capabilities: its horsepower. Based on that term, you may assume that a horse can produce around 1 horsepower. Linguistically, it makes perfect sense. In reality, however, it's way off the mark.

So, how much horsepower can one horse produce? And how did this term get started, anyway?

The maximum output of a horse is actually much closer to 15 horsepower, according to the University of Calgary's Energy Education website. In fact, a more befitting name for the unit might be "humanpower," given that the average healthy person can produce just over 1 horsepower.

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So where did the term come from, then? It was first coined in the late 1700s by James Watt, a Scottish engineer remembered for his iconic, and incredibly efficient, steam engines. Seeking a way to advertise the contraptions, he invented a unit of measurement that would effectively showcase the superiority of his steam engines compared with something people were familiar with: horses.

Watt determined — from personal observation rather than rigorous scientific study — that a working horse could turn a mill wheel 144 times each hour. Using this number, he estimated that horses were capable of pushing 32,572 pounds 1 foot per minute, or about 14,774.41 kilograms 1 meter per minute. For convenience, he rounded this up to 33,000 pounds foot-pounds of work per minute (14,968.55 kilograms), and the "horsepower" unit was born, according to Encyclopedia Britannica. 

Watt didn't care much about the accuracy of the measurement, only that it highlighted the drastic productivity improvements buyers would experience if they purchased one of his steam engines. His machines were indeed far more powerful and reliable than horses, and as a result, very few people questioned — or cared about — the veracity of his calculations. 

Watt was, by all accounts, an engineering genius, and was so revered by his peers that, in recognition of his pioneering deeds and endeavors, the "watt" unit of power was ultimately named after him in 1882. However, given that we now know a horse can exert far more than 1 horsepower, why do we still use a term Watt created as part of a marketing campaign? 

"Because of the way language is always changing, there are more words that are estranged from their origins than people might realise," said Eric Lacey, a senior lecturer in English language at the University of Winchester in the United Kingdom.

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"Sometimes, words don't look right because they're using older meanings," Lacey told Live Science in an email. "There is nothing tall or high about a 'highway' — this word comes from an older meaning of 'high' which was 'main,' and so a 'highway' is just a 'main road.' We have the same sort of thing in the phrase the 'high seas,' which just meant 'the main seas.'"

This type of confusion plagues many of our words for measurements.

"There are also a lot that just don't make sense anymore because, as with 'horsepower,' they were based on estimations that could be variable or misguided," Lacey said. "The measurement of an 'acre,' for example, was roughly the unit of land which could be plowed by one person with a single yoke of oxen in one day — roughly 4,426 square meters [47,641 square feet]. However, this could be higher or lower depending on the type of plow, the shape of the land and how demanding the overlord was."

A word's meaning can be corrupted or largely forgotten over time, but what makes a term stand the test of time? Why are some words consigned to the past while others, such as "horsepower," become ubiquitous?

"This is the holy grail of linguistics! If we could accurately predict what words would be ingrained in people's minds, we'd be earning a fortune providing them to the advertisement industry," Lacey said. "The crucial thing to bear in mind here is that the words people use are guided by two things: individuals, and the way they interact. 

Individuals may avoid words like slurs because they have problematic meanings or negative connotations, or may select words because they like their new meanings or how they sound, Lacey said.

"Secondly, individuals may select certain words because of the bigger picture of their various interactions — they might participate in social trends, or respond to cultural events or be trying to emulate the vocabulary of somebody they aspire to be like," Lacey said. On a group level, people may use words to signal their identities and values, to show they are up-to-date or to mock something. 

"Against this backdrop, we can see how a [culturally significant] word like 'horsepower' survived," Lacey said. "If horses hadn't been the most obvious sources of industrial energy in the early 19th century, it's doubtful the term would have been as popular, but the fact that a single word could both convey the desired redundancy of the old and simultaneously usher in the new meant it ended up at the forefront of everybody's minds."

When asked if he would be supportive of rebranding "horsepower" to "humanpower," Lacey said, "As a linguist, I'd be very happy with that!" It would be a neat example of a word actually telling people what it did and would be more meaningful as a unit of measurement, he added.

Originally published on Live Science.