The American colonies declared their independence from Britain in the same decade as Arkwright opened the first cotton mill and Smith published Wealth of Nations. Within around 60 years the United States population at over 17 million had overtaken Britain’s. Over the following 25 years it almost doubled, and doubled again in the following three decades. By 1880 the United States gross national product was already one and a half times that of Britain. By 1900, it was double Britain’s and by 1920, three times.