PHYSICS IN WARFARE 



97 



at the end of successive seconds will be denoted by the Roman 

 figures above the horizontal line. 



Let the equally spaced numbers below the horizontal 

 line represent the position of the sound-wave of the discharge 

 at the end of successive seconds. 



Let us consider the state of things at the end of six seconds. 

 The pressure-wave set up by the shell as it leaves the gun 

 will have reached the position indicated by the arc p, drawn 

 with centre O and radius 6. The sound-wave of the report 

 will also have reached the same position. As in the case of 



Fig. 4. 



the howitzer shell, the gun shell is continuously setting up 

 pressure-waves which travel onward with the velocity of sound, 

 and so slower than the shell itself. It is again impossible to 

 draw all these pressure-waves, and so, as before, we will content 

 ourselves by indicating those set up at the end of successive 

 seconds. 



At the end of the first second the shell will be at the position 

 /, its pressure-wave will have five seconds left in which to travel 

 until the time we are considering. It will, therefore, have 

 come into the position represented by the arc q, whose centre 

 is /, and whose radius is 5. 



