In 1839, during a visit to the Continent, Miss Martineau's health broke down. She retired to solitary lodgings in Tynemouth, and remained an invalid till 1844. Besides a novel, _The Hour and the Man_ (1840), _Life in the Sickroom_ (1844), and the _Playfellow_ (1841), she published a series of tales for children containing some of her most popular work: _Settlers at Home_, _The Peasant and the Prince_, _Feats on the Fiord_, &c. During this illness she for a second time declined a pension on the civil list, fearing to compromise her political independence. Her letter on the subject was published, and some of her friends raised a small annuity for her soon after.
In 1844 Miss Martineau underwent a course of mesmerism, and in a few months was restored to health. She eventually published an account of her case, which had caused much discussion, in sixteen _Letters on Mesmerism_. On her recovery she removed to Ambleside, where she built herself "The Knoll," the house in which the greater part of her after life was spent. In 1845 she published three volumes of _Forest and Game Law Tales_. In 1846 she made a tour with some friends in Egypt, Palestine and Syria, and on her return published _Eastern Life, Present and Past_ (1848). This work showed that as humanity passed through one after another of the world's historic religions, the conception of the Deity and of Divine government became at each step more and more abstract and indefinite. The ultimate goal Miss Martineau believed to be philosophic atheism, but this belief she did not expressly declare. She published about this time _Household Education_, expounding the theory that freedom and rationality, rather than command and obedience, are the most effectual instruments of education. Her interest in schemes of instruction led her to start a series of lectures, addressed at first to the school children of Ambleside, but afterwards extended, at their own desire, to their elders. 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 Charles Knight she wrote, in 1849, _The History of the Thirty Years' Peace, 1816-1846_--an excellent popular history written from the point of view of a "philosophical Radical," completed in twelve months.