412 Capt. Noble. Note on the Energy absorbed [Feb. 4, 



ISTow, if the results given in this table be examined, it will be 

 observed that the whole of the velocities obtained from the gun with- 

 out twist are higher than those obtained from the gun rifled with a 

 uniform twist, while the whole of the velocities obtained from the 

 last-mentioned gun are higher than those obtained from the gun 

 with the parabolic or uniformly increasing twist. 



Using the mean results, there is a loss of velocity of 20 ft.-secs. in 

 passing from the gun with no twist to that with a uniform twist, and 

 a further loss of 29 ft.-secs., or 49 ft.-secs. in all, in passing to the gun 

 with the parabolic rifling. Translating these losses of velocity into 

 losses of energy, it appears that there is a loss of 21 ft.- tons, or about 

 1'5 per cent, of the total energy due to the uniform rifling, and a 

 further loss of 39 ft.-tons, or 275 per cent., making 60 ft.-tons, or 

 about 4^ per cent., in all when the parabolic rifling is employed. 



In a paper published in vol. 45 of the 'Philosophical Magazine ' 

 (1873) I investigated the ratio existing between the forces tending to 

 produce translation and rotation in the bores of rifled guns, and I 

 showed that, if R be the pressure tending to produce rotation, and G 

 .be the gaseous pressure acting on the base of the projectile, the 

 resultant of which pressure acts along the axis of the bore, that is, 

 along the axis of Z, then in the case of the parabolic rifling 



B== 



where r is the radius of the bore, p the radius of gyration of the 

 projectile, k the principal parameter of the parabola (the plane of xy 

 being supposed to be at the vertex of the parabola and at right 

 angles to the axis of the bore), 8 the angle which the normal to the 

 driving surface of the groove makes with the radius at the point 

 under consideration, v the velocity at that point, y x the coefficient of 

 .'friction. 



While in! the case of a uniform twist 



R = _ 2*? 2 G- 



sin d 



where h is the pitch of the rifling, k the tangent of the angle which 

 the groove makes with the plane of xy, the other constants, &c., 

 bearing the; meaning I have already assigned to them. 



Now to obtain the numerical values of R from the above equations, 

 a knowledg^ of the values of Gr, that is, of the total pressures acting 

 Ion the base j of the projectile, and in the case of the parabolic rifling 

 jof the velocity at all points of the bore, is necessary, and, the explosives 



