Slide rule belonging to James Watt, Scottish engineer, 1790-1819.

This slide rule was part of the contents of the private workshop of James Watt (1736-1819), the Scottish engineer and instrument maker, at his house in Heathfield, Birmingham, where he lived from 1790 until his death. Watt invented the modern steam engine, which became the main source of power in textile mills in Britain during the Industrial Revolution. His engine had a separate condenser through which steam from the cylinder was pased and cooled, allowing the engine to be kept hot. This reduced fuel consumption and saved time. The SI (international standard) unit of power, the watt (W), is named after him.