It will be noticed that though guns now barely reach half the weight of the monster muzzle-loaders, they are even more effective. Thus the 46-ton (12-inch) gun hurls an 850-lb. projectile with a velocity of 2750 foot-seconds, and uses a comparatively small charge. The famous “81-ton” needed a very big charge for its 1700-lb. shell, and had little more than half the velocity and no such power of penetration. This change has been brought about by using a slower-burning explosive very powerful in its effects; enlarging the chamber to give it sufficient air space, and lengthening the chase of the gun so that every particle of the powder-gas may be brought into action before the shot leaves the muzzle. This system and the substitution of steel for the many layers of welded iron, makes our modern guns long and slim in comparison with the older ones. To resist the pressure of the explosion against the breech end, a tightly-fitting breech-plug must be employed. The most modern and ingenious is the Welin plug, invented by a Swedish engineer. The ordinary interrupted screw breech-plug has three parts of its circumference plane and the other three parts “threaded,” or grooved, to screw into corresponding grooves in the breech; thus only half of the circumference is engaged by the screw. Mr. Welin has cut steps on the plug, three of which would be threaded to one plane segment, each locking with its counterpart in the breech. In this case there are three segments engaged to each one left plane, and the strength of the screw is almost irresistible. The plug, which is hinged at the side, has therefore been shortened by one-third, and is light enough to swing clear with one touch of the handwheel that first rotates and unlocks it. The method of firing is this: The projectile lifted (by hydraulic power on a ship) into the loading tray is swung to the mouth of the breech and pushed into the bore. A driving-band attached near its base is so notched at the edges that it jams the shell closely and prevents it slipping back if loaded at a high angle of elevation. The powder charge being placed in the chamber the breech-plug is now swung-to and turned till it locks close. The vent-axial or inner part of this breech-plug (next to the charge), which is called from its shape the “mushroom-head,” encloses between its head and the screw-plug the de Bange obturator, a flat canvas pad of many layers soaked with mutton fat tightly packed between discs of tin. When the charge explodes, the mushroom-head--forced back upon the pad--compresses it till its edges bulge against the tube and prevent any escape of gas breechwards.