Mule power helped industry imagine what machine power could be. Mule culture, however, spread unevenly around the world. Equine mules are so rare in Britain that when English farmers talk of mules, they’re likely referring to a hybrid sheep. The absence of mule culture followed the English empire into the Americas; the Spanish monopoly on mules meant few worked colonial plantations in the Caribbean or British North America.

In marketing his steam engine to the muleless English territories, James Watt was strategic about comparing engine power to the country’s more relatable and treasured animal. Had Watt been Spanish or Portuguese, we could have had engines measured in mulepower.

Engines evolved around the concept of horsepower, but in practice, they often coexisted with mules. Even as innovation expanded the energy sources engines could harness, many remained tethered to mule power.

Science History Institute Museum & Library