To sum up, Lord Kelvin's scientific activity had lasted for nearly seventy years. He was born four years after Oersted made his famous discovery of the action of an electric current on a magnet, and two years before Ampère, founding on this experiment, brought forth the first great memoir on electromagnetism. Thus his life had seen the growth of modern electrical science from its real infancy to its now vigorous youth. The discoveries of Faraday in electrical induction were given to the world when Lord Kelvin was a boy, and one of the great tasks which he accomplished was to weave these discoveries together in a uniform web of mathematical theory. This theory suggested, as we have seen, new problems to be solved by experiment, which he attacked with the aid of his students in the small and meagrely equipped laboratory established sixty years ago in the Old College in the High Street. It was his lot to live to see his presentations of theory lead to new developments in his own hands and the hands of other men of genius--Helmholtz and Clerk Maxwell, for example--and to survive until these developments had led to practical applications throughout our industries, and in all the affairs of present-day life and work. His true monument will be his work and its results, and to only a few men in the world's history has such a massive and majestic memorial been reared. He was a tireless worker. In every day of his life he was occupied with many things, but he was never cumbered. The problems of nature were ever in his mind, but he could put them aside in the press of affairs, and take them up again immediately to push them forward another stage towards solution. His "green book" was at hand on his table or in his pocket; and whenever a moment's leisure occurred he had pencil in hand, and was deep in triple integrals and applications of Green's Theorem, that unfailing resource of physical mathematicians. Saepe stilum vertas quae digna legi sint Scripturus, the motto which Horace recommends, was his, and he would playfully quote it, pointing to the eraser-pad in the top of his gold pencil-case. He erased, corrected, amended, and rewrote with unceasing diligence, to the dismay of his shorthand-writing secretary. The theories and facts of electricity and magnetism, the production and propagation of waves in water or in the luminiferous ether, the structure and density of the ether itself, the relations of heat and work, the motions of the heavenly bodies, the constitution of crystals, the theory of music, the practical problems of navigation, of telegraphing under the sea, and of the electric lighting of cities--all these and more came before his mind in turn, and sometimes most of them in the course of a single day. He could turn from one thing to another, and find mental rest in diversity of mental occupation.