(p. 49) With his wife running the household and tending to their four-year-old daughter, Sally, Priestley simply had more time on his hands to explore, invent, and write. Priestley was retracing a pattern that Franklin had originally carved two decades before, when he handed over day-to-day operation of his printing business to his foreman, David Hall, in 1748 and then spent the next three years transforming the science of electricity. Necessity may be the mother of invention, but most of the great inventors were blessed with something else: leisure time.
Johnson, Steven. The Invention of Air: A Story of Science, Faith, Revolution, and the Birth of America. New York: Riverhead Books, 2008.