A curious series of experiments, made by Colonel Bomford of the United States Ordnance Department, told for the first time where the greatest strain was exerted on the bore of a gun, and gave some idea of the relative strain elsewhere along the bore. Taking an old cannon, he drilled a hole from the side near the muzzle directly into the bore and inserted a pistol-barrel. Then he put a bullet into the pistol-barrel, and after loading the cannon in the ordinary way, he fired it. Of course, as the cannon-ball was driven from the big gun the powder gas behind it drove the pistol-ball from the pistol-barrel. The colonel measured the velocity of the pistol-ball and made a note of it. Then he drilled another hole in the cannon some inches farther from the muzzle, and repeated the experiment. The force exerted on the pistol-ball was slightly greater there. By drilling other holes he learned approximately the pressure all along the bore. It appeared that the greatest pressure was directly over the shot when it was rammed home against the powder. From there the pressure decreased rapidly, being only about half as much when the ball had travelled four times its own diameter from its original resting place. [Illustration: A Dahlgren Gun.] Captain Rodman of the Ordnance Department, put a piston in place of Bomford’s pistol-barrel and let the piston punch into a piece of copper, and then determined the pressure on the piston by forcing the same kind of a piston into the same kind of a piece of copper by a known weight. “Although not an accurate process,” it was good enough, and with the figures obtained by it before him, Lieut. John Dahlgren, of the United States navy, designed the gun of smooth outline that by its splendid success in the hands of both forces, during the Civil War, made him famous. The greatest thickness of metal was placed around the greatest strain, and a proper thickness at every inch of length of the bore. It was not alone, however, in putting the metal where it would prove most serviceable that Dahlgren made his gun efficient. He was a metallurgist, and was careful to improve the quality of iron used. Meantime Captain Rodman had proposed to cast cannon hollow and cool them from the interior, instead of casting a solid log of iron and boring it out on the old plan. Although the first experiments did not show any especial improvement in the strength of a gun so cast, the method was eventually found to be the best. With Dahlgren’s model a gun of eleven inches of diameter of bore was cast in 1852. It was fired 500 times with shells and 655 times with solid shot that weighed 170 pounds, the service charge of powder being fifteen pounds. The gun was not seriously injured or worn even by the work. That settled the status of the Dahlgren guns, and from that time on they were furnished with reasonable rapidity to the new steam warships of the navy.