The method of construction is the “built-up” process, shown by the partial section on Plate V., the barrel being reinforced by tubes which are shrunk on—like the tire of a wagon-wheel—so as to produce initial compression. The explosion in the powder chamber strains and expands temporarily the barrel, and the application of the shrinkage principle enables a portion of the strength of the tubes to be employed in preliminary internal pressure. The barrel thus supported can be strained by the charge, not only to its own limit of safety, but to an additional amount equal to this initial compression. The all-steel, built-up gun has a possible rival in wire-wound ordnance, a system which replaces the tubes, to a greater or less extent, by layers of wire, wound while in tension around the barrel. Powder is the soul of the gun; it transforms the huge inert mass into a flaming engine of death. The great development of explosives began but a generation since. The researches of Robins and Rumford in the last century, and of Hutton in the dawn of this, formed the world’s knowledge of the gun’s internal ballistics until the year 1870. To the genius of Noble and Abel is due the stimulus to growth since then. The powders have kept pace with gun-construction in its advance. The increased strength of the chamber has been met by heavier and slow-burning charges—cocoa, brown prismatic, and the like—which have given not only greater initial velocity, but a continuous acceleration through bores whose maximum length has exceeded 47 feet. Indeed, to the production of this lingering combustion is due the great linear dimension and power of modern guns. Initial pressure had its limit; advance lay only in the subsequent acceleration given by late ignition of a portion of the charge. Gunpowder, however, after a reign of more than five hundred years, has been dethroned. The “villainous saltpetre” of the monk, with its allies, charcoal and sulphur, yields now to nitro compounds, which produce not only far greater energy, but are as well smokeless. The sea-fights of our war with Spain saw the last contending fleets to be wrapped in a cloud, lingering and baffling, of their own making. Cordite, one of these compounds in use abroad, is prepared in long “cords” from di-nitro-cellulose and nitro-glycerine. The new smokeless “powder” of the United States navy is made from nitro-cellulose dissolved in ether alcohol. France was the first in employing explosives such as these, which, in their offensive and tactical advantages, form one of the signal triumphs of the century’s last years.