DETONATION OF HIGH EXPLOSIVES OR BY THE IMPACT OF BULLETS. 449 



The percentage of momentum trapped by short pieces is 13 per inch, and the 

 corresponding maximum pressure for the normal velocity of 1240 feet per second is 



0-13x38 

 32 ; 2xO-98xlO- = 



The maximum pressure which should be exerted by a perfectly fluid bullet having the 



same mass and velocity is 



The time taken by the bullet to travel its own length is 8'4 x 10'* seconds. Thus 

 if the bullet were perfectly fluid, the whole momentum should be trapped in a piece 

 9 inches long, whereas in fact only 86 per cent, is so trapped. The errors inherent 

 in the method of experiment, which have been discussed in the last section, will all be 

 less at the lower velocity. On the other hand the rigidity of the bullet will IKJ 

 relatively more important and probably suffices to account for much of the difference 

 between the theoretical and observed times of impact. 



The 700 feet per second bullets showed a maximum pressure of 5450 Ibs., as 

 compared with 5320 Ibs. calculated. 54 per cent, of the momentum was trapped by 

 a 9-inch piece. It was not possible to experiment with longer pieces, so that the time 

 of impact in this case could not be determined. 



It should be observed here that just after the piece has been shot off it tends to 

 pull the rod after it by magnetic attraction, which of course still continues after the 

 joint is broken, though it diminishes rapidly as the distance between piece and rod 

 widens. The effect of this is to give more momentum to the rod and less to the piece 

 than they would respectively possess as the effect of the blow alone. By measuring 

 the amount of the magnetic pull when the piece is held at different distances from the 

 rod, the current in the solenoid being the same as that used in the impact experiment, 

 it is possible to estimate the amount of this effect. With 2000 feet per second bullets 

 it is quite negligible, but when the velocities are lower particularly with long pieces, 

 it necessitates a correction. This correction has been applied in the figures given 

 above for the 1240 feet per second and 700 feet per second bullets. 



Detonation of (run- Cotton. 



It is well-known that a charge of 1 Ib. gun-cotton will shatter a mild steel plate 

 1 inch thick or more, if it be detonated in firm contact with it. The fracture is quite 

 " short," like that of cast-iron, though the broken pieces are usually more or less 

 deformed. Typical fractures of this kind obtained on plates of very good mild steel 

 are illustrated in figs. 8, 9, 10, and 11. Figs. 8 and 9 are photographs of a plate 

 1 inches thick originally quite flat. It was broken by a slab of gun-cotton weighing 

 1 Ib. which covered the section of the plate AB and was detonated in contact with 



VOL. CCXIII. A. 3 M 



