In hydrodynamics also Lord Kelvin never lost sight of practical applications, even while pursuing the most intensely theoretical researches into the action of vortices or the propagation of waves. In his later years he worked out the theory of ship-waves with a power which has made more than one skilful and successful cultivator of this branch of science say that he was no mere mathematician, but a man who, like the prophets of old, could divine what is hid from the eyes of ordinary mortals. Of the ultimate importance of these for practical questions of the construction of ships, and the economy of fuel in their propulsion, there can be little doubt. Unhappily, the applications will have now to be made by others. It is interesting to note that the investigation of waves in canals with which Lord Kelvin recently enriched the _Proceedings of the Royal Society of Edinburgh_ have been carried out by a strikingly ingenious adaptation of the Fourier solution of the differential equation of the diffusion of heat along a bar, or of electricity along a slowly worked cable. Thus, beginning with Fourier mathematics in his earliest researches, he has in some of his last work applied the special exponential form of Fourier solution of the diffusion equation to a case, that of wave propagation, essentially different in physical nature, and distinct in mathematical signification, from that for which it was originally given. Lord Kelvin's written work consists of the _Electrostatics and Magnetism_, three volumes of _Collected Mathematical and Physical Papers_, three of _Popular Lectures and Addresses_, the _Baltimore Lectures_, a very considerable number of papers as yet uncollected, and the _Natural Philosophy_. But this, great as it was, represented only a relatively small part of his activities. He advised public companies on special engineering and electrical questions, served on Royal Commissions, acted as consulting engineer to cable companies and other corporations, was employed as arbiter in disputes when scientific questions were involved, advocated distinctive signalling for lighthouses and devised apparatus for this purpose, and he was, above all, a great inventor. His patents are many and important. One of them was for a water-tap warranted not to drip, another, for electrical generating machines, meters, etc., was perhaps the patent of largest extent ever granted.