Another problem of mechanical engineering is to determine whether it will be found more economical to transform the energy of coal, at the mines, into electric current and send it by wire to cities and other places where it is wanted, or to carry the coal by rail and water, as we now do, to such places, and convert it there by the steam or gas engine. In favor of the first method it can be said that hills of refuse coal now representing locked-up capital can be burned, and the cost of transportation and handling be saved. Electric energy can now transport power in high voltage economically between coal-mines and most large cities. The second method has the advantage of not depending on one single source of supply, that may break down, but in having the energy stored in coal-pockets near by the place of use, where it can be applied to separate units of power with no fear of failure. It seems probable that a combination of the two systems will produce the best results. Where power can be sent electrically from the mines for less cost than the coal can be transported, that method will be used. To prevent stoppage of works, the separate motors and a store of coal, to be used in cases of emergency, will still be needed, just as has been described as necessary to the commercial success of wave-motors. ELECTRICAL ENGINEERING Any attempt by the writer of this article to trace the progress of electricity would be but a vain repetition, after the admirable manner in which the subject has been treated in a former paper of this series by Professor Elihu Thomson. We can only once more emphasize the fact that it is by the union of four separate classes of minds—scientific discoverers, inventors, engineers, and capitalists—that this vast new industry has been created, which gives direct employment to thousands, and, as Bacon said 300 years ago, has “endowed the human race with new powers.” METALLURGY AND MINING All the processes of metallurgy and mining employ statical, hydraulic, mechanical, and electrical engineering. Coal, without railways and canals, would be of little use, unless electrical engineering came to its aid. It was estimated by the late Lord Armstrong that of the 450,000,000 to 500,000,000 tons of coal annually produced in the world, one-third is used for steam production, one-third in metallurgical processes, and one-third for domestic consumption. This last item seems large. It is the most important manufacturing industry in the world, as may be seen by comparing the coalless condition of the eighteenth century with the coal-using condition of the nineteenth century.