And as to methods of measurement, those who remember the haziness of even telegraph engineers as to the measurement of the efficiency of electrical currents and electromotive forces in the circuits of lamps and dynamos, in the early days of electric lighting, know how much the world is indebted to Thomson.[26] He it was who showed at first how cables were to be tested, as well as how they were to be worked; it was his task, again, to show how instruments were to be calibrated for practical measurement of current and energy supplied by the early contractors to consumers. He had in the quiet of his laboratory long before elaborated methods of comparing resistances, and given the Wheatstone balance its secondary conductors for the comparison of low resistances; he now showed how the same principles could be applied to measure the efficiencies of dynamos and to make up the account of charge and discharge for a secondary battery. And if the siphon-recorder and the mariners' compass and the sounding machine proved pecuniarily profitable, the reward was that of the inventor, who has an indefeasible right to the fruit of his brain and his hand. But Lord Kelvin's activity was not confined merely to those practical things which have, to use the ordinary phrase, "money in them"; he gave his time and energies freely to the perfecting of the harmonic analysis of the tides, undertook again, for a Committee of the British Association, the investigation of the tides for different parts of the world, superintended the analysis of tidal records, and invented tide-predicting machines and improved tide-gauges. Lord Kelvin's work in the theory of heat and in the science of energy generally would have given him a title to immortality even if it had stood alone; and there can be no doubt, even in the mind of the most determined practical contemner of the Carnot cycle, of the enormous importance of these achievements. Here he was a pioneer, and yet his papers, theoretical and yet practical, written one after another in pencil and despatched, rough as they were, to be printed by the Royal Society of Edinburgh, form, as they are collected in volume i of his _Mathematical and Physical Papers_, in some respects the best treatise on thermodynamics at the present time! There are treatises written from a more general standpoint, which deal with complex problems of chemical and physical change of means of thermodynamic potentials, and processes which are not to be found set forth in this volume of papers; but even these are to a great extent an outcome of his "Thermoelastic, Thermomagnetic and Thermoelectric Properties of Matter."