The Flight of a Bullet 75 



spark show that at speeds above 1,100 feet per second the bullet 

 is accompanied by a conical pressure-wave analogous to the 

 "bow and stern" waves spreading from a steamer which is going 

 through the water faster than the waves she makes. It is the 

 production of this conical pressure-wave which is responsible for 

 the increased resistance to the bullet's motion. It is also the 

 cause of the " crack " of the rifle, the sharp sound heard as the 

 bullet passes, which precedes the duller sound of the explosion 

 coming from the muzzle. 



The necessity for spinning the bullet arises from the fact 

 that without spin elongated bodies moving through a resisting 

 medium tend to set themselves broadside on to the motion. 

 This is seen when pieces of paper or leaves fall through the air, 

 or when a shell or flat stone is dropped through water. The 

 bullet at first travels along the direction of its axis and takes the 

 wind fairly on its point. But as it rises its axis becomes slightly 

 inclined to its direction of motion, and the wind strikes it under 

 its point. The air flowing past the bullet in this inclined position 

 is much more checked in its motion round the point than past 

 the base. In consequence af this the pressure of the air is greater 

 at the point than in the rear of the bullet, so the point is pushed 

 up and the axis comes to be across the line of motion. This is 

 what happens in the absence of spin. When the bullet has a 

 sufficiently rapid spin it does not " upset " in this way, but its 

 axis " precesses," i.e., rotates in a narrow cone round the direc- 

 tion of the motion. The motion is identical with that of a peg- 

 top, which would fall if it were not spinning, but which can carry 

 out this conical motion of its axis ("precession ") when its rota- 

 tion round its axis is sufficiently rapid (An experiment with a 

 gyrostat was shown). In the case of the bullet the effect is to 

 keep it moving nearly point forward throughout its flight. 



(3) The effects produced on the body struck vary greatly 

 with the physical nature of the medium in which it is brought 

 to rest. Some of the observed results are of a rather paradoxi- 

 cal nature. For example, a metal bullet can be shattered by 



