If the muzzle velocity in each case is multiplied by 

 this ratio it will give what may be considered as the 

 average velocity of the escaping gases as they leave the 

 muzzle. Taking the muzzle velocity m each case as 1 ,230 

 ft. per sec, this gives the velocity of the gases as 2,680 

 ft. per sec. for black, and 3,910 ft. per sec, 3,840 ft. per 

 sec, and 3,690 ft. per sec respectively for the three types 

 of smokeless powder. It will be noticed that all these 

 velocities are much above the velocity of sound, which 

 is about 1 , 1 00 ft. per sec. 



Now, when the velocity of the expanding gas is greater 

 than 1 ,100 ft. per sec, the surrounding air is piled up in 

 front of it, forming a dense envelope, which is driven 

 outward by the high pressure gas inside. As the gas 

 expands its pressure falls until it is below that of the at- 

 mosphere, but the envelope of the compressed ciir still 

 travels outwards and further diminishes the pressure of 

 the gas immediately behind it. The disturbance then 

 becomes a wave of compressed air followed by a wave 

 of rarefied air, and after travelling a certain distance this 

 settles down to a velocity of 1,100 ft. per sec, and is 

 recognised as a sound wave. The charader of the sound 

 depends upon the nature of the impulse given by the 

 original explosion. 



An air-gun makes practically no noise, because the 

 muzzle pressure is very low ; a pop-gun makes much more 

 noise, because the muzzle pressure is considerable. A 

 small pile of loose gunpowder makes very little noise upon 

 being fired, because the pressure, and consequently the 

 velocity, of the gases is lov/. The same amount of ful- 

 minate of mercury makes a very sharp stinging report, 



71 



