James Watt (1736-1819). Caricature of the Scottish engineer and inventor James Watt. Watt made numerous practical improvements to the steam engine of his day. By 1790, the superior Watt steam engine had completely replaced the older Newcomen variety. His inventions included the double acting engine, where the piston is made to both push and pull (1782), and the pressure gauge (1790). He also adopted the centrifugal governor to regulate the speed of a steam engine and coined the term 'horsepower' to describe a unit of energy.