The Scottish inventor wrote that to a friend a few years before patenting something that would change the world forever.
In 1781, the steam engine came to light.
Actually, the first engine of James Watt had only 10 horsepower and more than ten years the steam finally pushed the big locomotives (with a thousands of times greater power).
How did it work?
The steam was heated by burning wood and it increased its pressure. This pressure was used to push the locomotives with a mechanical system.
Even today we use a lot of steam. Indeed, the nuclear power plants use this fluid to move electric turbines, with the difference that the steam is heated by nuclear reactions.