give Botation to Rifled Projectiles. 213 



26. From an examination of the values of R given in this Table, 

 it will be seen that the total pressure on the driving-surface 

 reaches about 31 tons shortly after the commencement of 

 motion, and the projectile quits the bore with a pressure of 

 about 36 tons. With the view of making the variations which the 

 pressures undergo more readily comparable, I have laid down in 

 Plate VI. the curves derived from equations (15) and (17) for 

 the battering charge of pebble-powder. 



From these diagrams the pressures on the driving-surface 

 at any point of the bore, both for the uniform and parabolic 

 twists, can be seen by simple inspection. The line of abscissae 

 gives the travel of the shot, and the ordinates give the corre- 

 sponding total pressure on the studs. 



The curves show that with the uniform spiral the pres- 

 sure on the studs reaches nearly 70 tons after a travel of -3 

 feet, rapidly falling to about 9 tons at the muzzle, while with 

 the parabolic rifling the pressure at *3 feet of travel, correspond- 

 ing to the point of maximum pressure, is only 31 tons. The 

 pressure then falls slightly and amounts to 28 tons at about 1 

 foot travel ; thence it gradually increases to a maximum of 36 

 tons at the muzzle. 



By way of comparison, I have added in the Plate a curve 

 showing the pressures required to give rotation to a 400-lb. 

 projectile fired from the 10-inch gun with uniform twist when 

 R. L. G. instead of pebble-powder is used. 



The curve in this case is of the same nature as that derived 

 from the pebble-powder ; but the variation is greater, the maxi- 

 mum pressure being much higher and the muzzle-pressure, 

 owing to the smaller charge, somewhat less. 



27. To one more point it is worth while to call attention. 



If the gun were a smooth-bore gun, the equation of motion 

 would be 



M -S= G '; < 24 > 



and comparing this equation with (10), we have, on the sup- 

 position * that the velocity increments in both cases are equal, 



G=G'+R-{ v 4s > n S S) 2 + * 2 + ?*?+*»}' " (25) 

 or, in the case of the Woolwich gun, where 8=90°, 



G = G ' +R -(^S5t} ; • • (26) 



* Were the velocity increments not supposed equal, the reduction of 

 pressure due to the suppression of rifling would be less than that given in 

 the text. 



