EVOLUTION OF THE MODERN HEAVY GUN. 157 



Passing now to the construction of the modern gun, a longi- 

 tudinal 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 suflBciently 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. Th& 

 hoops therefore serve to re- enforce the jacket by their own tend- 

 ency 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 ex- 

 aggerated 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 



cutive or tuisTtc ntiisTJtHcz 



Curves showing Decrease of Elastic Eesistance, Powder Pressure, and Increase ob 



Projectile Velocity. 



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 sea- 

 coast, siege, or field artillery, are breech-loading. Many futile- 



