2o:> ARTILLERY 



tion round those points, or, in other words, round the minor axis. Thus several os- 

 cillating motions are set up, the force of which varies with the amount of the obstacle 

 and the suddenness of the force applied ; i.e., with the amount and nature of the 

 powder-charge. Moreover, in the case of the increasing spiral, further mechanical 

 forces are brought into operation at the muzzle, which have escaped Captain Noble's 

 investigation. An ever-changing angle of groove cannot bo conformed to by a constant 

 angle of shot, any more than a male screw of one pitch can work into a female screw 

 of a different pitch. To meet this obvious mechanical difficulty, only one stud, one 

 inch long, bears in each groove all along the bore ; but when within twelve inches 

 of the muzzle, an angle of twist is reached which coincides with the angle formed by 

 the front and rear studs on the shot. Now as this is the spot where most of the 

 " increasing spiral " guns give way, a mathematical examination of the forces brought 

 into play near the muzzle would be fraught with much interest. But these and 

 other mechanical inquiries find no place in Captain Noble's formula. He assumes 

 that the original action on the stud is a pressure, not a blow, and that this pressure 

 follows a uniform law throughout the gun. Granting this singular hypothesis, the 

 conclusions are rather of a philosophical than practical character, dealing, as we have 

 said, with but a small fraction of the force of translation, and a quite insignificant 

 increment of gaseous pressure. Subject to those deductions, his conclusions are worthy 

 of note. 



' Captain Noble educes from his formulas what is, we doubt not, the fact, that in a 

 uniform twist the pressure on the studs is a constant fraction of the pressure of the 

 base of the shot, the value of the fraction depending on the angles of the rifling. 

 The tension of the powder-gases at the muzzle being very small whom compared with 

 their tension at the seat of the shot, the studs have, on the uniform system, scarcely 

 any work to do at the muzzle, while they may be severely strained at the commence- 

 ment of motion. He estimates the pressure on the studs due to rifling as, in this 

 case, about 2 per cent, of that required to impart translation to a 400-lb. shot in the 

 10-inch 18-ton gun, or about 68 tons' pressure when it has moved four inches, 

 and 9 tons at the muzzle ; whereas the substitution of the parabolic curves for 

 the uniform angle of spiral, according to Captain Noble's formulas, reduces this 

 pressure one-half, so that when the stud has moved four inches (i.e., before it touches 

 at all on the " driving" side), it sustains 31*2 tons' pressure, gradually rising to 36 

 tons at the muzzle ! 



'As Sir William Armstrong certifies that "the maximum pressure" (in the powder- 

 chamber) " causes the failure of the stud," the difference of powder-pressure arising 

 from a change of spiral is noteworthy. Captain Noble tells us that, small as the in- 

 crement in gaseous pressure due to rifling is, it is still less in the parabolic than in the 

 uniform spiral. Whereas the maximum bursting pressure is reduced from 19'7 tons 

 per square inch to 19'5 tons per square inch, by suppressing the rifling altogether in 

 the case of uniform spirals, the decrement of pressure due to the suppression of the 

 parabolic rifling is a reduction from 19'7 tons to 19'62 tons per square inch. The 

 gain then, to the powder-chamber, from the employment of the increasing spiral, is 

 12 of a ton per square inch. We commend this philosophical fraction to our artillery 

 philosophers, and would make them a present of this mathematical advantage. When, 

 however, we turn to the Tables of Pressures registered by the Committee on Explo- 

 sives in the 10-inch 15-ton, which is the subject of Captain Noble's learned investi- 

 gation, we are rather puzzled to which of the pressures we are to apply the '12 of a 

 ton. We find similar powder-charges fired under identical conditions producing most 

 unlike results. We find these anomalous pressures with every description of powder ; 

 and we observe that the gun has this parabolic system of rifling, with its consequent 

 stud agency. Yet, with these great philosophical advantages, the powder-pressures 

 registered in the 10-inch gun, with 87 Ibs. P. charges and 400-lbs. shot, varied from 

 26 tons on the square inch to 63'4 tons, the latter expulsive force resulting in the least 

 velocity and striking force in the projectile. Again 60 Ibs. E.L.G. charges registered 

 powder-pressures varying from 36'5 tons to 57'8 tons on the square inch, under 

 identical conditions, the highest expulsive force imparting the lowest velocity to the 

 projectile. Yet it is on the register of pressures within this gun that Captain Noble's 

 calculations are based ; true, he does not select any of the above figures for his 

 formulas, but on the pressures registered with 85 Ibs. P. charge, which happened to be 

 1 9'7 tons on the square inch. But, if a selection must be made between pressures varying 

 from 19'7 tons to 63'4 tons on the square inch, it might bo quite as well to close the 

 Keport of the Committee on Explosives, and assume any number of tons at haphazard. 

 ' Captain Noble has shown a mathematical gain of '12 of a ton pressure on the square 

 inch by the adoption of the parabolic or increasing spiral. What if the greater part 

 of this astounding variation of pressure from 19'7 to 63'4 tons on the square inch was 

 attributable to the- parabolic curve, or, to epeak more accurately, to the stud system 



