Her interest in schemes of instruction led her to start a series of lectures, addressed at first to the school children of Ambleside, but afterward extended to their parents at the request of the adults. The subjects were sanitary principles and practice, the histories of England and North America, and the scenes of her Eastern travels. At the request of the publisher Charles Knight, in 1849 she wrote "The History of the Thirty Years' Peace, 1816–1846", an excellent popular history from the point of view of a "philosophical Radical". Martineau spanned a wide variety of subject matter in her writing and did so with more assertiveness than was expected of women at the time. She has been described as having an "essentially masculine nature". It was commonly thought that a "progressive" woman, in being progressive, was improperly emulating the qualities of a man.