PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 



403 



metal about the breech, that the fracture would occur through the cascabel, 

 where the metal has more than twice the thickness exhibited in the army 

 columbiad, but this principal direction is usually the result. 



Guns sometimes exhibit additional fractures to those represented above, 

 and this occurs when the thickness of metal is continued further forward 



towards the muzzle, having the same 

 effect as if a tire, or strong band, 

 were placed upon the gun at the 

 place where the fracture usually 

 branches off to either side, thus 

 delaying the longtitudinal fracture 

 until the expansion lengthwise of the inner metal is greater than the 

 elaHticUy and duciiUty of the re-enforce, when the cross fracture occurs. It 

 may be said then, in brief, that the fractures at right angles to the plane of 

 the bore are caused by the lengthening of the inner metal about the bore 

 by heat, while the outer metal remains the same length, or with less 

 expansion of length, until ruptured, and that longitudinal fractures are due 

 principdhj to the enlargement of the inner metal hy heat in the direction of 

 the diameter, or radiaUy. 



If the gun be parallel all the way to the muzzle, the cross fractures will 

 occur more frequently along the re-enforce, because in that part it is 

 exposed to the highest temperature, and, consequently, the greatest expan- 

 sion of length. I have seen a diagram of an Armstrong gun with only one 

 r~\ fracture. In this gun a die is 



pressed with a powerful screw 



fl juWm^ J _ ~ ~ — against the inner metal of the 



1 Q Jlllliiin: „J p-un, and against the bottom of 



the bore. A heavy weight on the 

 crank is used, by repeated blows, 

 to press forward the die by the screw with considerable force. The screw 

 and die were made of steel with little compressibility^ and the lengthwise 

 expansion of the inner metal of the gun would increase the tension upon 

 the re-enforce, already great from the pressure of the screw, to bursting. 

 Greater thickness of metal, at either end of the re-enforce, would make the 

 cross fracture more frequent. It is a corroboration of this theory that the 

 guns of the Dahlgren model, with more than double the thickness of metal 

 behind the chamber, though made of the strongest material, should break 

 in the same direction, forward of the trunnions, but sometimes exhibit only 

 cross fractures (when made of cast iron) at the rear of the trunnions.^ It is 

 evident that the model is best in which the direction of the fracture is least 

 uniform, but a properly constructed gun should not burst at all. 



Guns, however, are usually broken through the breech— their strongest 

 part— and beyond the range of the pressure, which is, of course, lim- 

 ited to the bottom of the bore or chamber. The diagram in Capt. Rod- 



_1 



S!ig.6- 



ZJ 



man's book, p. 43, exhibiting the 

 various kinds of strain to which a 

 gun is subjected at each discharge, 

 considers the gun as if made up of 

 staves, and really exhibits only the 



