Complex sentence: In 1834, after completing the economic series, Harriet Martineau paid a long visit to the United States during which she visited a great many people, some little known, others as famous as James Madison, the former US president, at his home at Montpelier.
Split into simpler sentences: In 1834, after completing the economic series, Harriet Martineau paid a long visit to the United States. During this time, she visited with James Madison, the former US president, at his home at Montpelier.