Page:Popular Science Monthly Volume 88.djvu/883

 Popular Science Monthly

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��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 modern 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 moving 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.

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