ON RIFLED GUNS. 27 



inch, was elongated 0, 0002171th part of its entire length, and that where the si rain exceeded 

 this, the metal took a now sot and altered its mplecular structure, for beyond 5000 it did not 

 recover the dimensions of which it had been deprived. Moreover, it appears, at page 1(5(5 of 

 same work, that the repeated application of a tensile strain equal to 15000 to the square inch, 

 gave a constantly increasing extension, showing a decreasing power of resistance. Thus, the 

 first application gave an extension 0,00127S; the 100th, 0,001510; and the 250th, 0,001537. 

 The moduli, computed from these data, are for the 



1st, M = 11737100 Log = 1,0095000 



100th, M= 9933770 Log = 6,9971 144 



250th, M== 9759270 Log = 6,9894174 



and computed from the extension 0.0002171, produced by 5000 pounds, 

 M = 23030S00 Log = 7,3623102 



Every variation in the modulus requires a corresponding variation in the value of c, 



Eq. (13). And that a gun may be uninjured by firing, the value of M — should never exceed 



P 



the maximum tension from which the gun may recover its dimensions ; and keep its 

 molecular structure unaltered. This, in the present case, is 5,000 pounds, and which in Eq. 

 (10) will give, in the case of any gun made of this iron and whose radius and thickness are, 

 respectively, p and c, the maximum gas pressure to which such gun should be exposed. This 

 prescribes a rule for the treatment of guns in actual service. 



(22) — If the exigencies of an occasion require a greater pressure, then the gun, after a 

 certain number of rounds, should be thrown aside, broken up and recast. 



(23) — But if it be the question to construct a gun to bear a given gas pressure, and of which 



the material will bear the tensile strain M — without change of molecular structure, we have 



P 



onlv to substitute the given pressure P and the tensile strain in Eq. (13), to find the necessary 

 thickness. 



It is quite apparent that all guns have their limits of endurance, and that these limits are 

 somewhat narrow. Those charged with their use have ever at hand the means of pushing 

 them beyond these limits, and these means they are very apt to apply unless restrained by 

 well defined and very positive rules. These rules require a knowledge of the relations which 

 connect the proportions of the constituents of powder, density, size of grain and volume, with 

 the pressure which the gas arising from its combustion exerts upon the bore of the gun. 

 This knowledge we do not possess. In the first part, upon the strains of rifled guns, were 

 given the objections, founded upon the principles of mechanics, to Major Rodman's inferences 

 from his plug experiments in this regard. 



(24) — In support of those objections, the following table, giving the results of a series of 

 experiments by Mr. Parrott, with one of his 100-pounders, is appended: 



(339) 



