PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 547 



enlargement of the bore of the Ames gun at the 450th round, as mentioned 

 in the i)ami)hlot " on the inejfficiencxj of heavy ordnance," by Wiard, page 21. 



THE EFFECT OF ENLARGEMENT OF THE BORE. 



■33. The parallelism of the bore of a wrouglitiron g-nn is continually 

 changing, alfecting its range and accuracy — both disabling the gvin and 

 preparing it for an explosive bursting, by communicating to it initial ten- 

 sion, by enlarging the interior; in which state, if it is afterwards fired 

 rapidly, it will burst like any other gun. 



34. A gun is heated nearly the same with small charges of powder as 

 with heavy, because, with a heavy charge the shot is ejected somewhat 

 quicker, and, although the temperature of the gases is highei-, the time of 

 action is less. 



ENDURANCE OF CAST-IRON. 



35. Cast-iron lias heretofore exhibited greater endurance of surface to 

 the. bore in guns than any other -material used. A 30-pounder Parrott fired 

 on Morris Ii^land endured 4, GOG rounds before it burst, with a minimum 

 abrasion of surface of bore and enlargement of vent. Its small she, thin- 

 ness of wall, heated through more uniformly than a gun of thicker walls; 

 and being fired slowly with intervals of fifteen minutes between the rounds, 

 it was not exposed to the rupturing principle hereafter to be more particu- 

 larly described. The only enlargement of bore occurring in a cast-iron 

 gun being within the limits of its elasticity, it recovers its original size and 

 form after the force acting upon it has been expended. If the forces acting 

 upon it are great enough to extend it beyond the limits of its elasticity, it is 

 ruptured, having but little ductility. It will be shown that these qualities 

 are most desirable in guns, and that cast iron can be so used as to be 

 absolutely safe from bursting, whether by heavy charges or by rapid firing, 

 when of course its endurance of surface and cheapness is so much in its 

 favor that it must be the material from which heavy guns are to be made 

 hereafter. 



36. All guns of large sizes are subjected to severe strains from unequal 

 heating or cooling, at two important periods viz: when they are being fab- 

 ricated and when they are used in service. 



DNEQUAi; COOLING OF CAST-IRON. 



37. A fifteen-inch Rodman gun, cast hollow, and cooled from the exterior to 

 get initial tension, at Pittsburg, a few months since, when nearly cold and 

 ready to be removed from the pit, split from end to end. The inner metal 

 had been cooled by water passing through the bore while the outside metal 

 retained a higher temperature. By this means Colonel Rodman attains 

 initial tension in his gun, as in the hooped guns of Parrott, Armstrong, 

 Blakcly and Brooke. (See fig. 3.) 



The outside metal cooling at a later time shrinks upon the inner metal 

 like the tire upon a wagon wheel, but in the case referred to the tension 

 was too great. The cast block was about sixteen feet in length, and the 

 wall about, sixteen inches in thickness — consequently the area of cross 

 section ruptured was equal to 3,172 inches. If the tensile strength of the 

 metal was 30,000 lbs. to the square inch of section, the pressure bearing 



