ON RIFLED GUNS 



29 



greatest tensile strain from which the material may recover its former figure after being 

 relieved. In the case of the Greenwood iron 42-pounder, this is 5000. We find 



P = 21165 lbs. 



(28) Again, let it be required to find the thickness of a 42-pounder to be subjected to a 

 gas pressure double this, and yet the tensile strain not to exceed 5000. Then Eq. (13) 



c = l'.0625, 

 which is the thickness necessary to resist a pressure equal to twice 21165 without incurring a 

 tensile strain greater than 5000 pounds. It is quite apparent how these numbers will vary 

 with the nature of the gun metal. In the testing of a new gun, means should be provided for 

 measuring the pitch of the sound with which it responds to a blow from a light hammer on the 

 face of the muzzle after every discharge. Altered tone will indicate altered structure. 



(29) — To determine the gas pressure let A B C D represent the 

 edge of a block of copper; KGH the cutter of the plug apparatus 



in the position to which it is pressed by a dead weight, without ^ — - 

 velocity, so that the resistance of the copper may measure the weight. *jf 

 Make this notation, viz: 



"W = Weight equal to copper resistance. 



x = Depth of penetration, G O. 



y = Distance E F, or length of cut. 



<p = Angle H G K. 



A = Copper resistance for unit of penetration. 



(30) — Assume as the law which connects the resistance with the penetration, 



W = Aa; m , (15) 



in which m is a constant, as well as A; both to be found by experiment. For any other 



weight as Wi and penetration x lt we have 



W I = A. x L m (16) 



Taking logs, in both equations, and subtracting first from the second, we find, after solving 



with respect to m, 



Log Wj — Log W 



and from Eq. (15) 



From the figure 



or 



Log xi — Log x 



W 

 A: 



w 



'x m 



(17) 

 (13) 



y = 2 x tan J <f 



y 



2 tan J <p 



which substituted for x in Eqs. (17) and (18) gives, after reduction, 



Log Wi - Log W 

 Log yi — Logy 



(19) 



(20) 



(341) 



