The Story of Science is a great six part BBC documentary that talks a lot about the phenomenon you are describing, and episode four talks about Watt specifically. I highly recommend it, and it can be found online.

http://www.dailymotion.com/video/x1xxn3o_bbc-the-story-of-sc...

http://www.bbc.co.uk/programmes/b00s9mms

http://en.wikipedia.org/wiki/The_Story_of_Science:_Power,_Pr...