Page:Popular Science Monthly Volume 51.djvu/167

Rh Passing now to the construction of the modern gun, a longitudinal section shows an inner tube rifled within and slightly enlarged at the breech end of the bore. Around this is a long tubular jacket extending from the breech two thirds of the length of the gun. Around this jacket is a series of compressing hoops, and around this a second or outer series of the same. Originally the interior diameter of the jacket is a little less than the exterior diameter of the tube. By heating the jacket sufficiently it is made to expand until it can be slipped over the cold tube, which becomes enormously compressed by the subsequent cooling of the jacket. In like manner the first hoop is too small to be slipped over the cold jacket except when heated for this purpose. The same remark applies to the second hoop. The final result, as shown by the cross-sectional diagram on opposite page, is that the diameters of the tube, both internal and external, are permanently diminished by the compression of the jacket, while those of the hoops are permanently increased. Their contractile force is not sufficient to compress the jacket, which is itself resisting the enormous reacting force of the compressed tube within. The hoops therefore serve to re-enforce the jacket by their own tendency to contract from the enlarged condition in which they were applied while hot. They are in a state of permanent tension. The scale of differences exhibited in the diagram is greatly exaggerated to make these perceptible. The longitudinal diagram shows by curves how the expansive force of the exploding powder diminishes from breech to muzzle, how the yet greater elastic



resistance of the steel components, after they are assembled together, is adjusted to resist this expansive force, and how the velocity of the projectile increases from breech to muzzle.

All rifled guns built in America at present, whether for seacoast, siege, or field artillery, are breech-loading. Many futile