PHYSICS IN WARFARE 99 



The intervals between these breaks varied from string to 

 string according to the situation of the microphone. The 

 interval in each case is represented by the length AB in Fig. 

 4. In the case of a gun, therefore, the film looked something 

 like that shown in Fig. 5. 



The interval between the onde-de-choc and the gun report 

 is an indication of the calibre of the gun, for the bigger the 

 gun the faster does the shell travel, and hence the greater 

 this interval. 



The calibre of the gun may, however, be determined in a 

 different way. 



Let us suppose that the positions of the gun and of the burst 

 have been determined accurately, and are located at G and B 

 respectively on the map of the district used by the computers 

 in the field. Let M be one of the microphones. The 

 calibre of the gun can be determined if the range and 

 time of flight are known. The range is determined merely by 

 measuring the distance GB on the map. The determination of 

 the time of flight is somewhat more difficult. 



Suppose the gun is fired at zero time, which is not observed. 

 Let the sound reach the microphone, M, x seconds later, this 

 being found by measuring the distance GM by a ruler graduated 

 to read the results in seconds of time. From the film we find 

 that the sound of the burst reaches the microphone later still, 

 say y seconds afterwards, so that the sound of the burst reaches 

 the microphone at a time x plus y seconds. But the burst 

 occurred before this time, say, z seconds previously, a time 

 found by measuring the distance MB in seconds as before, 

 so that the burst occurred at a time x -{- y — z seconds. 



Since the gun was fired at zero time, the time of flight must 

 be X -{- y — z seconds. 



The calibre of the weapon is then found from tables. 



It remains to be added that, when the hostile artillery had 

 been located, our own batteries often fired on it. In such cases 

 the Sound-rangers would frequently pick up the bursts of our 

 own shells, and so indicate where they had fallen with respect 

 to the target. 



