DISCOVERY 



21 



at the spot H.Q. about four to sLx thousand yards 

 from the line. Here the whole of the apparatus for 

 locating the hostile batteries was kept except the sLx 

 sound-detectors, or, as they are also called, the micro- 

 phones. One of these instruments was placed at 

 each of the points a, b, c, d, e, and F, which points 

 were usually from three to five thousand yards behind 

 the front line. Each of these points was carefully 

 surveyed so that its exact position on the map was 

 known. Usually it was arranged that the si.x points 

 were at intervals of about a mile, and lay on a circle, 

 the centre of which was inside the enemy's lines at 

 about the average distance back of the enemy's heavy 

 artillery. This was done merely for convenience, and 

 is not a necessary element in locating the batteries. 

 The sound-detectors at each of these six points were 

 connected up with the recording apparatus at H.O., 

 by wires, just as though they were telephones. It was 

 found that it was much better in every way to detect 

 the report of a German gun with an apparatus than by 

 the ear of a man. The very instant the sound was 

 heard at b, for example, the fact was instantaneously 

 communicated along the wires to H.Q., where it was 

 recorded. If a man, however, was obser\-ing, a time of 

 one-twentieth to a fifth of a second would elapse be- 

 tween his hearing the sound and his pressing a button 

 to record it, depending upon his experience and his 

 natural quickness. This is a very interesting fact 

 in experimental psychology, but in sound-ranging it led 

 to inaccuracies. A sound-detector, specially designed 

 by one of the officers, was therefore used instead. 



There were observers, however, at the observation 

 post, marked O.P. Their duty was to listen, and to 

 observe. When a German gun or howitzer fired, one 

 of them pressed a button as soon as he heard the 

 report. This had the effect of starting up the whole of 

 the recording apparatus at H.O. A few seconds later 

 the report of the gun would be heard by the micro- 

 phone which was nearest to the battery firing, then by 

 the second nearest, and so on till all had heard it. 

 When the observer considered that the sound had 

 passed over aU the detectors, he ceased pressing his 

 button, and so stopped the recording apparatus work- 

 ing. This remained quiescent until the German 

 battery fired again. The observer next telephoned to 

 H.O., in cheerful tonei, the direction from which the 

 sound of the enemy gun had come, his idea of the size 

 of the shell, and, when he could see what the enemy 

 were trying to hit, the approximate location of the 

 target. All this information is very valuable to the 

 men at H.O., whose job it is to work out the location 

 accurately. 



I have said the forward observer at O.P. controls the 

 recording apparatus at H.Q. This observer is suffi- 

 ciently in front of the Hne of detectors to hear most 



[Continued on p. z3 



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