The gristmills at the Upper Falls would have used a system developed in the 1790s by Oliver Evans, who combined existing technology with machines of his own invention to grind, dry, sift, and pack finished flour into barrels — the first automated assembly line. The process, for which he received one of the nation’s first patents, reduced waste and the amount of labor required to mill grain. In his 1795 book describing the new process, “The Young Mill-Wright & Miller’s Guide,” he calculated the savings at a double mill (one with two pairs of millstones) at $298 a year. “A mill that made 40 barrels a day, required four men and a boy,” he explained. “Two men are now sufficient.” (Library of Congress.)