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 a number of experiments on the heating of a disc by rapid rotation in vacuo, the apparatus for which was designed by R. Beckley, engineer to the Kew observatory. The experiments have been discussed by [q. v.], Professor Ludwig Boltzmann, and others, but no adequate explanation of the heating effect has yet been given. In 1865–1868 Stewart published, in conjunction with Warren de la Rue [see ] and Benjamin Loewy, a long series of investigations on sunspots, the variation of which they attempted, though without decided success, to trace to changes in planetary configuration. Stewart showed, however, that the daily range of magnetic variation appeared to be connected with these changes. He spent much effort from this time until his death on the discovery of certain periodic inequalities in terrestrial and solar phenomena, and attempted to deduce causes for these inequalities; but these deductions, as Stewart knew, can only be regarded as valid when based on an extremely large number of observations ; and, together with William Dodgson, William Lant Carpenter, and other coadjutors, he spent a large amount of labour on the necessary calculations.

On 1 Jan. 1867 he was appointed secretary to the government meteorological committee, and in this and the following year he supervised the installation of meteorological stations all over the kingdom. He resigned the post in 1869. On 7 July 1870 Stewart was appointed professor of natural philosophy in the Owens College, Manchester, a post which he retained till his death. He continued, however, to act as superintendent of Kew observatory till 1871. In one of his journeys from London, in November 1870, he met with a railway accident in which his thigh was crushed, and for nine months lay ill at Harrow, in the course of which he passed from ‘vigorous activity … to a grey-headed old age,’ although his mental powers remained unimpaired.

In April 1875 was published anonymously a book called ‘The Unseen Universe’ by Stewart and his friend Professor Tait, in which the authors aimed at deducing from the combination of a number of theological postulates with current scientific doctrines the existence of the soul and of a transcendental universe. The book is written in a popular and picturesque style, and excited much attention, running through fourteen editions in thirteen years. The authors avowed their identity in the fourth edition (April 1876). Professor William Kingdon Clifford (1845–1879) made an attack on the book from the heterodox point of view in the ‘Fortnightly Review’ (June 1875), to which a reply was offered in the preface to the second edition. A sequel published in 1878 by the same authors, dedicated to the members of the Paradoxical Society, and entitled ‘Paradoxical Philosophy,’ portraying in dialogue form the conversion of a cynical and heterodox German mathematician to religious and social orthodoxy, proved less successful.

Stewart, who was a devoted and fervent churchman, was elected by a conference held at Lambeth Palace on 7 Jan. 1881 as member of a committee for promoting interchange of views between scientific men of orthodox views in religious matters. He was also one of the founders of the Society for Psychical Research, in whose investigations he took a deep interest; he made several short contributions to its proceedings, and was president of the society from 1885 till his death.

In February 1887 he was elected president of the Physical Society, and also of the Manchester Literary and Philosophical Society. He died on 19 Dec. 1887 of apoplexy, at Ballymagarvey, a small estate near Drogheda, which he had inherited and whither he had gone to spend his Christmas vacation. He married, on 8 Sept. 1863, Katharine, only daughter of Charles Stevens, a lawyer in London. Two sons and a daughter survived him. Stewart was a man of exceptionally modest, gentle, and kindly nature. A photograph of him is in the common-room of the Owens College.

According to the bibliography by Professor Schuster in the ‘Memoirs of the Manchester Literary and Philosophical Society,’ Stewart published about sixty-seven papers of his own composition exclusively. In collaboration with others, he published two papers with J. Brito Capello, astronomer at Lisbon, three with W. L. Carpenter, ten with De la Rue and B. Loewy, two with the Rev. Father Walter Sidgreaves of Stonyhurst, two with Professor Tait, four with William Dodgson, one with Morisabro Hiraoka, three with B. Loewy, one with Father Stephen Joseph Perry [q. v.] on the comparison of magnetic observations at Kew and Stonyhurst, and one with (Sir) Henry Enfield Roscoe. He also contributed various reports to the British Association.

In addition to the papers and books already mentioned, Stewart published a number of successful text-books, which are not only in general conscientious and accurate, but show considerable power of picturesque illustration. Their titles are:
 * 1) ‘Treatise on Heat,’ 1866; 3rd edit. 1866; 5th edit. 1888.
 * 2) ‘Lessons in Elementary Physics,’ 1870.
 * 3) ‘The Conservation of Energy,’ 1872, a popular ex-