We had better go back with it and learn something about the manufacture of a big gun. Guns used to be cast as a solid chunk of metal. Now they are built up in layers. To understand why this is necessary, we must realize that steel is not a dead mass, but is highly elastic--far more elastic than rubber, although, of course, it does not stretch nor compress so far. When a charge of powder is exploded in the barrel of a gun, it expands in all directions. Of course, the projectile yields to the pressure of the powder gases and is sent kiting out of the muzzle of the gun. But for an instant before the shell starts to move, an enormous force is exerted against the walls of the bore of the gun, and, because steel is elastic, the barrel is expanded by this pressure, and the bore is actually made larger for a moment, only to spring back in the next instant. You can picture this action if you imagine a gun made of rubber; as soon as the powder was fired, the rubber gun would bulge out around the powder-chamber, only to collapse to its normal size when the pressure was relieved by the discharge of the bullet. Now, every elastic body has what is called its elastic limit. If you take a coil spring, you can pull it out or you can compress it, and it will always return to its original shape, unless you pull it out or compress it beyond a certain point; that point is its elastic limit. The same is true of a piece of steel: if you stretch it beyond a certain point, it will not return to its original shape. When the charge of powder in a cannon exceeds a certain amount, it stretches the steel beyond its elastic limit, so that the bore becomes permanently larger. Making the walls of the gun heavier would not prevent this, because steel is so elastic that the inside of the walls expands beyond its elastic limit before the outside is affected at all. Years ago an American inventor named Treadwell worked out a scheme for allowing the bore to expand more without exceeding its elastic limit. He built up his gun in layers, and shrunk the outer layers upon the inner layers, just as a blacksmith shrinks a tire on a wheel, so that the inner tube of the gun would be squeezed, or compressed. When the powder was fired, this inner layer could expand farther without danger, because it was compressed to start with. The built-up gun was also independently invented by a British inventor. All modern big guns are built up. HOW BIG GUNS ARE MADE