214 Pressure required to give Rotation to Rifled Projectiles. 



and the interpretation of these equations is that the gaseous 

 pressure in the rifled guns (rifled with the parabolic twist) is 

 greater than that in the smooth-bored gun by the second term 

 of the right-hand member of the equation. 



28. The corresponding equations for a uniform twist are 



or, if 8=90°, 



G=&+U {v^h} • • (38) 



29. 1 shall now put these results in actual figures, and shall again 



take for illustration the 10-inch gun, supposed (as before) to be 



rifled, 1st, on the uniform, 2nd, on the parabolic or service twist. 



With the uniform twist, G (see Table) =1547 tons; and 



using equation (28) and the values of the constants given in 22, 



G'=G--245R 



= -989G . . (29) 



Hence the decrement of pressure due to the suppression of 

 rifling is only about 1 per cent. ; that is, the total pressure on 

 the base of the shot is reduced from 1547 tons to 1530 tons, or 

 the bursting pressure is reduced from 19*7 tons per square inch 

 to 19' 5 tons per square inch. 



At the muzzle of the gun in the same manner we find that 

 the total pressure is reduced from 205 tons to 202*8 tons, and 

 the pressure per inch in a corresponding proportion. 



30. Similarly, from equation (26) and the values of the con- 

 stants given in 25, the values of G' at the point of maximum pres- 

 sure and at the muzzle of the gun are obtained ; and I find that 

 with the parabolic twist the pressure on the base of the shot 

 would be reduced from 1547 tons to 1541 tons, or the bursting 

 pressure would be reduced from 19'7 tons to 19*62 tons per 

 square inch. 



At the muzzle the corresponding reductions are from 205 

 tons total pressure, to 196 tons, or from 2'61 tons to 2*49 tons 

 per square inch. 



31. For the sake of clearness, I recapitulate the results at which 

 I have arrived. They are as follow: — 



1st. That the pressures actually exerted at all points of the 

 bore to give rotation to the 10-inch British- service projectile, 

 compared with the pressures which would be exerted were the 

 gun rifled on a uniform twist, are very approximately exhibited 

 in the diagrams on Plate VI. 



2nd. That in the 10-inch gun (and other guns similarly rifled) 



