James Watt can be seen in this later French print working on John Anderson's model Newcomen engine. This work inspired Watt to invent his separate condenser.
Image: Science Museum/SSPL

Eureka!

Watt's breakthrough came on a Sunday afternoon walk in Glasgow in May 1765. He recalled that:

'...The idea came into my mind, that as steam was an elastic body it would rush into a vacuum, and if a communication was made between the cylinder and an exhausted vessel, it would rush into it, and might there be condensed without cooling the cylinder... I had not walked further than the Golf-house when the whole thing was arranged in my mind.'