In such a ship, deep down within the fortress is the massive and complicated machinery, steam and electric, upon which the life and activity of the whole structure depend. The power is generated in four enormous boilers, seventeen feet in diameter and twenty in length, their steel shells one and a half inches thick, built to carry a working-pressure of 160 pounds to the square inch. Each pair of these boilers, placed fore and aft and side by side, is installed in a separate compartment, with fire-rooms at the ends. Every boiler has four furnaces in each end, which give eight to each fire-room, or a total of thirty-two. The two boiler compartments are separated by a water-tight bulkhead, and by a deep, broad coal-bunker. At the sides of the ship are also coal-bunkers, which supplement the heavy armor-belt by the protection of a mass of coal twelve feet in thickness—in itself a not inconsiderable earthwork, which might arrest the fragments of a bursting shell that had succeeded in piercing the armor. No casualty of naval combat can be worse than the penetration of high-pressure boilers by heavy shells. Their complete protection is an imperative condition, quite as important as the protection of the magazines. Such is a modern battle-ship—a “wonderful and complex instrument of warfare,” as Lieutenant Staunton has expressed it. She is filled [he tells us] with powerful agencies, all obedient to the control of man—the creatures of his brain and the servants of his will. Steam in its simple application drives her main engines and many auxiliaries. Steam transformed into hydraulic power moves her steering-gear and turns her turrets. Steam converted into electrical energy produces her incandescent and search-lights, works small motors in remote places, and fires her guns when desired. Every application of energy, every device of mechanism, finds its office somewhere in that vast hull, and the source of all the varied forms of power lies in the great boilers, far down below danger of shot and shell, under which grimy stokers are always shoveling coal. Decades of thought and study, experiment and failure, trial again with partial success, and repeated trials with complete success, have assigned to each agency its appropriate function, and perfected the mechanism through which its work is performed. These modern developments have added one entirely novel and tremendous adjunct to the fleet, in the torpedo-boat and its terrible weapon. These take the place to some extent of the fire-ship of a century ago, which was designed to injure the enemy not by silencing his guns or overcoming his gunners, but by insidiously destroying his ship itself.