Page:Popular Science Monthly Volume 88.djvu/881

 Why Does a Rifle Crack?

��By Edward C. Grossman

��A WAR strength infantry company lay in our rear. We walktsl toward its far-off target, nearly in the line the bullets would take, a few yards' divergence to the left giving us the safety margin w^e felt would be enough with such expert marksmen. From some indefinite point in the air to our right, there came a sudden burst of high, thin, eerie crashes, the thin crash that comes from the leap of the electric spark from the static ma- chine, repeated in fitful fashion. Most extraordinarily, the sound lacked any definite point of origin ; it seemed higher than we were; and it seemed to come from our right. Nearer than this we could not locate it. A slight lull in the sharp crackling, and there came another sound — the heavy, dull thudding of guns fired at a great distance. As we progressed toward the long fire target twelve hundred yards from the infantry, the queer crackling noise followed us, growing thinner and more weird, but the thudding of the far distant guns grew fainter.

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��PhotOKraph by Or- 1 nance Dcparlinrnt. V,. S. N.

The bullet was photographed when six inches from the muzzle, just escaping from the blast gases of the rifle. Note the outline of the sound wave diverging from the nose of the bullet. This is the first stage of the exit of a bullet from a rifle's muzzle. It is not unlike the bow wave of a boat

���Photograph by Ohlnanco Depart

��This picture was taken when the bullet was eight inches from the muzzle, and traveling at a speed of roughly a half-mile a second. The two wires making the contact and the electric flash by which the photographs were made are shown as two black lines. The bullet takes its own picture. Note the eddying effect of the air behind the bullet. The fastest mechanical shutter, giving an exposure of one-thousandth of a second, would allow the bullet to move 2.7 feet during the opening of the lens

We were walking not more thai,! one hundred or one hundred and fifty yards from the line of fire of a trained infantry company, delivering its fire at a group- target twelve hundred yards, roughly three-quarters of a mile, away.

The thin, high-pitched crackling, that seemed at one time like the leap of the high-tension spark of the static machine, at antjtlier like the cracking of whips, and again like the vicious crash of a stone through glass, came from the flying bullets of the United States service rifle, which starts with the speed of twenty-seven hundred feet per second. The thudding, that fell oft' to almost nothing at twelve hundred yards, came from the rifles themselves, the only sound one hears when close to them, but the least noticeable at a distance when one is close to the course of the bullet.

As we gained the target, a new sound

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