194 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1916. 



familiar little whirlpool was created. It attracted the eye and 

 finally the mind, since there was a hole through which water was 

 passing but slowly, notwithstanding the fact that the drain plug 

 was removed. In a flash the analogy was apparent. It was obvious 

 that centrifugal force prevented the water from passing through the 

 hole rapidly. If the powder gases in a gun were given the same 

 vigorous Avhirling action, they would also acquire centrifugal force, 

 and, if their outlet hole were located at or approximately at the 

 center, they would exit relatively gradually. The}^ simply could not 

 exit until they had slowed down at least a little. The search was 

 ended. 



A little gas whirling device was quickly made and adjusted to the 

 barrel of a rifle and the first shot fired was the first quiet rifle shot 

 ever discharged from a high-power rifle. 



Wlien shooting was done in several different places, it began to 

 be apparent that the noise depended upon the place, at least when a 

 high-power rifle w^as used. It seemed to be impossible to eliminate a 

 certain sharp " crack." The character of this crack was similar to a 

 whiplash crack. It was entirely different from the more dull boom 

 of the report. By accident it was found one day that this " crack " 

 noise existed a long way down the range. A listener located at the 

 500-yard mark on a 1,000-yard range, detected the crack noise ap- 

 parently overhead. This indicated immediately that it was con- 

 nected wdth the bullet flight in some manner and was entirely sepa- 

 rate and apart from the report noise. 



Tests were made to bring out additional facts, and some of these 

 are instructive. It was suspected that the bullet flight created a 

 bow wave, creating a little zone of compressed air which moved out 

 from the trajectory, and that this wave was heard by reflection. The 

 person shooting the gun always heard a different noise from the per- 

 son located at a distant point down the range. A terrain was selected 

 on the extensive meadows on the Connecticut Eiver bank below 

 Hartford, where a series of clumps of bushes and small trees existed. 

 There were three separate clumps in front of which the bullet from 

 a Springfield service rifle could be made to pass. AVlien the gun was 

 fired, the listener at the gun heard three separate sharp cracks, and 

 a low rattle of many minor cracks. This pointed fairly conclusively 

 to the fact that the bow wave was reflected back from each of these 

 clumps, and separate noises were heard from each, because they were 

 separated by enough distance to give a distinguishable interval. 



It was then thought that firing down a railroad track which ran 

 along the open meadow, and had telegraph poles at regular intervals, 

 would giA-e a good test. This was done, and the result was a rapid 

 succession of cracks, just as had been anticipated. 



