Popular Science Monthly 



855 



mally, slips through the air like a trout 

 through water. 



The soldier, fired on and missed by a 

 single sniper without other sound to 

 confuse or cover up that pertaining to 

 him, hears two distinct sounds, if the 

 firing takes place within four hundred 

 yards or so. Pho- 

 netically they are 

 "Pack -p u n k." 

 The first is a 

 vicious and m_en- 

 acing crash. It is 

 the bullet arriving 

 with its regards to 

 him; the second 

 is the report of the 

 rifle which follows 

 along some dis- 

 tance behind the 

 bullet. The mod- 

 ern bullet travels 

 faster than does 

 sound, which has 

 but the speed of 

 eleven hundred feet 

 per second. The 

 person watching 

 the jet of steam 

 from the whistle of 

 the far-off locomo- 

 tive and noting the 

 interval of time. 

 which elapses 

 before the whoop 

 of the whistle 

 arrives, will ap- 

 preciate that sound 

 is a leisurely 

 traveler. 



The crash comes 

 from a vacuum 

 formed in the rear 

 of the flying bullet 

 by its enormously 

 quick displacement 

 of air. The bad 

 shape of the missile 

 allows the air to 



Drawings from photographs of bullets in 

 flight. Showing older type, metal jacketed, 

 small bore military bullet in flight. Note 

 bow wave of air driven ahead of the bullet, 

 and the eddies of air in the wake like 

 water in the wake of a ship. Directly in 

 rear of the base of the bullet is the vacuum 

 that causes the sharp crash as the air 

 closes suddenly in upon it 



The flight of the modem spitzer bullet, 

 which is used by Germany, England, 

 France, the United States and some other 

 nations. Note the sharper angle of the bow 

 wave, and the greater vacuum in the rear of 

 the bullet. This is caused by the fact that 

 these lighter sharp-point bullets are driven 

 at far higher velocity than the older type, 

 and the vacuum is more pronounced. Also 

 the noise is more marked. A bullet which 

 tapered down to the stem as sharply as the 

 point of the bow would have little vacuum 

 and little noise. The photograph from 

 which this sketch was prepared was made 

 by Professor Boys by means of an electric 

 spark produced as the bullet cut the wire 



flow back again around the stern, like 

 water around the stern of the fast mov- 

 ing boat. Finally, the air rushes in be- 

 hind the bullet and makes the crash 

 just as the air rushes in behind the elec- 

 tric spark. 



Only at speeds higher than twelve 

 hundred or fourteen hundred feet per 



second is this sound heard. Strangely 

 enough it is not heard if the bullet has 

 started at very high speed and falls to 

 this lower one. Possibly what is heard 

 in such case is the crash of the bullet at 

 some distance farther back where the 

 velocity is still high enough to produce a 

 crash. 



Military rifles 

 drive their bullets 

 at speeds of from 

 two thousand to 

 three thousand feet 

 per second. The 

 same bullets, load- 

 ed to give velocities 

 of less than four- 

 teen hundred feet 

 per second, do not 

 make a sound. So, 

 black-powder or 

 low-power rifles 

 like the familiar 

 .22, do not pro- 

 duce this crash 

 from their bullets. 

 The difterence in 

 the arriving time 

 of the two sounds, 

 bullet crash and 

 report of the rifle 

 which fired it, is 

 very noticeable at 

 the long ranges. 

 At one thousand 

 yards, for instance, 

 the bullet of the 

 United States rifle 

 arrives at the mark 

 1 .86 seconds after 

 it leaves the muzzle 

 of the rifle. The 

 bullet thus covers 

 the distance at the 

 a\'erage speed of 

 about sixteen hun- 

 dred feet per 

 second . Sound, 

 tra\ cling at the 



uniform rate of eleven hundred feet per 

 second, takes 2.7 seconds to make the 

 trip, and the bullet and its accompanying 

 crash, thus arri\e nearly a second ahead 

 of the report of the rifle. So comes 

 about the phenomenon of the two 

 distinct sounds; first the bullet crash, 

 and then the report of the rifle. 



