98 



SCIENCE PROGRESS 



In the same way the pressure-wave set up at the end 

 of the second second will be in the position r, centre //, 

 radius 4. 



Similarly, succeeding pressure-waves will be in the positions 

 s, /, u, the respective centres being ///, IV, and V, and respec- 

 tive radii 3, 2, 1. 



The waves we have drawn overlap at certain points. If 

 we had drawn all the waves set up at every instant we should 

 find a continuous overlapping, so that the true front of the 

 whole pressure-wave is evidently the envelope of the waves 

 we have indicated. 



The apparent effect observed by a person standing at some 

 such place as P is that he hears first of all a sharp crack due 

 to this envelope, then, after an interval, he hears the sound 



Fig. 5. 



of the discharge. The sharp crack is known technically as the 

 Onde-de-choc, and is followed by the true sound-wave at an 

 interval represented by AB expressed in time. Thus, in the 

 case of a gun, the observer hears two distinct reports, provided 

 he is standing in a suitable position. 



On the film the double break shows up distinctly, some- 

 what as indicated in Fig. 5. 



In locating the gun, therefore, the second set of breaks, 

 due to the true sound-wave, had to be used. 



It is clear that the onde-de-choc will be heard only by 

 an observer if he is standing somewhere between X and Y, 

 i.e. in what is known technically as the "Onde-de-choc cone," 

 a definite cone determined by the angle XOY. 



In general, most or all of the microphones were situated 

 in this cone, so that a double break was recorded by most or 

 all of the strings. 



