192 Mr. Mallet on the Physical Conditions 



without fracture, 4000 blows, each causing a deflection of one half the ultimate 

 deflection due to rupture from dead weight ; but no bar was broken by 4000 

 blows each producing a deflection of one-third this ultimate deflection : the blow 

 in every case being made with a very moderate velocity. 



110. As the valueof J, for impulsive forces (Eq. 17, 18) is double that for static 

 pressiu'e, it follows from the above that no cast-iron gun, whose proportions are 

 such that the mean extension of its metal, due to the maximum mean pressure 

 per square inch, of the explosion, exceeds one-fourth the extension due to the 

 strain of static rupture of the material, can withstand 4000 discharges. 



111. In the experiments made of a similar character on wrought-iron, how- 

 ever, such a limit for fracture was not reached. We may conclude, therefore, that 

 the number of rounds capable of being fired without final dislocation, from guns 

 similarly proportioned as to ultimate strength in relation to the eflFects of the 

 charge, will be much larger in wrought-iron than in cast-iron guns ; — assuming, 

 however, that in large wrought-iron guns, the physical properties of the wrought- 

 iron are the same as those of small rolled or forged bars, which is far from being 

 the fact, if they be forged in large masses. 



112. In the Great Exhibition of 1851 were several cast-iron guns, produced 

 at the Liege Foundry, Belgium, which were certified to have withstood the 

 following number of rounds respectively : — 



Size. Weight, lbs. Eomids. 



30-pounder, 6055 .... 2000 



24-pounder, short, . . . 1985 .... 3649 



6-pounder, 1954 ... . 6002 



6-inch howitzer, .... 1147 .... 2118 

 Several of the siege guns, 24-pounders, used at St. Sebastian in 1813, are 

 stated to have been fired 6000 rounds ; long before which, however, the vents had 

 been burnt away, and replaced, extemporaneously, with brass melted into them. 

 The mere statement, however, that a particular gun, or one of a particular 

 metal or casting, has stood a given number of rounds, proves nothing as to the 

 superiority or otherwise of the material, for the number of rounds that a gun 

 will stand is dependent, for guns of similar form and proportions, upon — 

 1°. The coefficients Te and Tr of the material. 

 2°. The excess of absolute spare strength of the gun measured in terms 



