Complex sentence: In 1834, after completing the economic series, Harriet Martineau paid a long visit to the United States, during which she visited with James Madison, the former president, at his home of 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 president, at his home of Montpelier.