horsepower

The wheel was to have been turned by horsepower, but it was adapted to be driven by a mill-wheel on the river Derwent […]

In the past, before the widespread adoption of SI units, the work that engines were capable of doing was compared with the work that horses could do – hence the term ‘horsepower’. Various people came up with various equivalencies, but the modern agreed definition is that 1 horsepower is 746 joules per second or 746 watts.

political horsepower