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 refractor, but it can now be realized that the plan of his work was unfortunate in conception. In choosing the subject of stellar parallaxes for investigation he was following in the footsteps of his predecessor at Dunsink, F. F. E. Brünnow, but instead of confining his programme to a small number of stars, he spread a wide net in the hope of discovering, by relatively few observations, such stars as might give evidence of exceptional proximity. As the working list included a high proportion of red giants, for reasons natural at the time but now understood to be particularly illusory, it is not surprising that the results were purely negative. From 1880 onwards Ball’s activity as a visual observer declined. His right eye, which first gave serious trouble in 1883, became quite useless within the next ten years and was removed in 1897; in the second place, other occupations began to take a larger share of his time.

At Cambridge, where he held a fellowship at King’s College, Ball’s academic lectures were highly appreciated, and he undertook there his last important work, a Treatise on Spherical Astronomy (1908). While at Dunsink he had installed a reflecting telescope for photographic work, the mirror being presented by Dr. Isaac Roberts [g.v.], and the Cambridge observatory under his direction received an important addition to its equipment in the shape of a photographic refractor in the coudé form.

The other occupations to which allusion has been made took two forms, and were mainly responsible for Ball’s reputation in the world at large. Between 1877 and 1906 he published no less than thirteen popular works on astronomy. Of these the most considerable and the most successful was The Story of the Heavens (first edition, 1886). But it was as a popular lecturer that he came in contact with the widest circle. In this capacity Ball showed all the qualities which make for success. He delivered courses of Christmas lectures at the Royal Institution, and for many years he lectured under the auspices of the Gilchrist Trust. Lecturing tours also took him to America in 1884, 1887, and 1901.

But Ball’s real reputation will not rest on his achievements as a popularizer of science, great as they were, nor even as an astronomer, in which capacity he lacked the advantages of professional training, though his energy and enthusiasm went far to supply the deficiency. It must be based on his work as a mathematician, in which he found his most absorbing interest and to which he devoted much of his leisure. Here he was fortunate in finding early a topic which gave a unity to all his researches. This was the theory of screw motions and their relations. Two books, The Theory of Screws: A Study in the Dynamics of a Rigid Body (1876) and A Treatise on the Theory of Screws (1900), incorporated at different stages the results obtained in a series of twelve great memoirs, published mainly by the Royal Irish Academy, of which he was secretary from 1877 to 1880 and a vice-president from 1885 to 1892. Even after the formal treatise, in the period extending up to his seventieth year, four additional memoirs were published on the same subject. Ball was a geometrician rather than an analyst, as would be expected of one who had received his early training in the Dublin school of his time. He has been ranked by Professor E. T. Whittaker as ‘one of the two or three greatest British mathematicians of his generation’. His presidential address to Section A of the British Association in 1887 was notable and characteristic.

Ball’s activities and interests were most varied. He was president of the Royal Astronomical Society from 1897 to 1899 and of the Mathematical Association in 1899 and 1900. He was scientific adviser to the Irish Lights Board from 1882 till his death, and rarely missed the annual tour of inspection by the commissioners. He took the most active interest in the affairs of the Royal Zoological Society of Ireland, of which his father had been honorary secretary; he was elected president in 1890 and held the office until he left Dublin for Cambridge. From his father also he had inherited a love of botany, and took a delight in the gardens attached to the observatories at which he resided. At Dunsink in particular, where the grounds are extensive, he indulged a hobby for practical farming, and experimented on pasture land with artificial manures, at that time a comparatively new method in Ireland. Though mainly English by descent, in appearance he was the typical Irishman of convention, and his geniality and sense of humour, which were always combined with shrewdness, made him universally popular. He was knighted in 1886. He died at Cambridge 25 November 1913.

Ball married in 1868 Frances Elizabeth, daughter of W. E. Steele, afterwards director of the National Museum of  19