Excerpts from Books and Wikipedia
"The real breakthrough came when James Watt, a Glasgow instrument-maker, found a way of dispensing with the hot-cold routine that had made previous steam-engines so inefficient. While repairing the University of Glasgow’s model Newcomen engine, he saw a way of employing all the steam generated: ‘the idea came into my mind that, as steam was an elastic body, it would rush into a vacuum, and, if a communication were made between the cylinder and an exhausted vessel, it would rush into it, and might be there condensed without cooling the cylinder.'" [Blanning: Pursuit of Glory, p. 132]