The general idea as regards the powder used as a propellant was that the ignition was instantaneous, and that the more violent the explosion the greater would be the velocity of the projectile. Under such conditions short weapons naturally found favour; and indeed, with a light, spherical, ill-fitting projectile, there was very little advantage to be gained by lengthening the bore. But with the introduction of rifled cannon, much heavier and better-fitting shot became possible, and a rapid-burning powder gave rise to dangerous pressures in the gun. It was then realised that it was not an explosion that was wanted, but a continuous pressure acting on the base of a shot for a relatively considerable period. This needed a slow-burning explosive, and led to the manufacture of powder as pebbles or prisms; the enlargement in the late 'seventies of the chamber of the gun, and the provision of air spaces for the expansion of the powder, greatly added to the velocity with which the shot left the gun, and therefore augmented its carrying power.[62] Gun-makers had meanwhile improved the strength of the weapon by a recognition of the fact that wrought iron was twice as strong in the direction of the fibre as across it; and thus in the 'sixties they began to coil the central tube, surrounding it by hoops, welded or shrunk on. The full advantages of fibre were thus secured for resisting circumferential strain. The bore was rifled to give the shot that rotatory motion which prevents irregularity in flight and conduces to accuracy of fire at long range. The smooth-bore gun was effective up to only 1000 yards range, as compared with the 6000 yards and 7000 yards for the modern weapon. Breechloading was first introduced into the Navy in the 'sixties, but discarded because the details for closing the breech end proved unsatisfactory. Finally, it was reintroduced in 1878, a satisfactory mechanism having been devised. These various improvements gradually increased the power of the gun. The length and weight had enormously grown, as is shown by the particulars of successive large Naval guns, shown in Table IV. on the next page; but the increase in energy up till the 'eighties was not commensurate with the augmentation of the weights of the projectile and charge. The advance from the 38-ton gun of 1870 to the 110-1/2-ton gun in 1887 involved the multiplying by five of the charge of powder, which quadrupled the energy of the gun, but the carrying power of the shot was still deficient. The velocity had increased in twenty years from 1600 to 2000 ft. per second, slower-burning powder having been introduced. TABLE IV. PARTICULARS OF THE SUCCESSIVE LARGE NAVAL GUNS, 1800 TO 1905.