22 



DISCOVERY 



gun reports two or three seconds before any detector 

 can. During this interval he has started up the 

 apparatus, and it is running merrily and is ready to 

 record. As each detector gets the sound, a little wire 

 at H.Q. gives a jerk, and the moment of this jerk is 

 recorded by photographing these wires on a moving 

 film. Evci"y thing is so arranged that simultaneously 

 the time intervals between these jerks are recorded on 

 the same film accurately to one-hundredth part of a 

 second. 



When the apparatus stops, the film is developed and 

 fi.xed in about ten seconds, and handed to a computer, 

 who obtains from it the time intervals between the 

 signals. These intervals arc. of course, the differences 

 in time between the sound of the gun arriving at the 

 sbc detectors. These give him five time differences — 

 namely, the differences between the sound arriving at 

 A and 13, B and c, c and d, d and e, and e and f. From 

 these five differences, after correcting for temperature 

 and wind, he gets his five lines, from the five lines his 

 intersection on the map, and the point of this inter- 

 section very approximately is the enemy gun. This 

 location is at once telephoned to the Heavy Artillery-, 

 who immediately take such action as they consider fit. 

 The sound-ranger's work is diagnosis. He says where 

 the offending battery is, whether or not it is very active, 

 what place it is trying to hit, and so forth. Our 

 artillery supply the remedj-. 



The accuracy of sound-ranging locations could be 

 tested in several ways. The results could be compared 

 with those obtained at the same time by other sources, 

 as, for example, by the observations of R.F.C. pilots 

 during the day, and of "flash-spotters" by night. 

 Secondly, the locations could be checked by comparison 

 with air photographs. This was most easily done 

 when some sign of an emplacement or of a battery 

 position was visible on the photograph. The third and 

 most convincing check was an examination of the 

 former German battery positions after an advance 

 had been made into their territory. Fifty yards was 

 about the mean error made in a sound-ranging location 

 under normal circumstances. 



A sound-ranger's life on a quiet front was a happy 

 one. On such fronts there was little enemy firing, and 

 the enemy did not move his batteries more than he 

 could help. The sound-ranging locations, collated 

 with evidence from air-photos and flashes at night, 

 led in general to the discovery of the exact positions 

 of the batteries. 



Very rapidly the special stunts, habits, and even 

 idiosyncrasies of each battery were known. They 

 became quite old friends. Such friends had a rough 

 time of it. As soon as they had opened fire, and in due 

 course been located, they were fired on by our artillerj-. 

 Anyone who knows about artillery firing will appreciate 



the difficulty and expense of " knocking-out " a hostile 

 battery, but it was comparatively an easy matter to 

 "silence" or " neutralize " it — that is to say, in plain 

 English, to make it stop. 



On such fronts, in addition to his normal work of 

 locating batteries, the sound-ranger could also 

 " observe " for the Artillery, if by " observe " we mean 

 here directing fire, not by looking at where the round 

 fell, but by listening for its burst. A great deal of 

 this work was done for our Heavy Artillery in 1917 

 and 1918 on all except the extremely noisy or battle 

 fronts, and much valuable aid was thereby given them. 

 A numerical example will help to make clear the method 

 by which this was done. Suppose a German battery 

 fired from such a position that its report arrived at 

 the detector at A first, then half a second later at b, 

 then one second later than that at c, one and a half 

 seconds later than that at d, two seconds later than 

 that at E, .and two and a half seconds later than that 

 at F. Suppose next that from these time differences 

 the sound-ranging section has located the position 

 of the offending gun. One of our own batteries, a 

 6-inch, an S-inch, or a 9'2-inch howitzer battery, wais 

 then called up from H.Q., and asked to fire on the 

 located point. In due course the round was fired and 

 exploded in enemy territory. This explosion was now 

 located just as though it were a gun report. Now, 

 if this noise arrived at B after A by exactly half a second, 

 at c after b by exactly one second, at d after c by one 

 and a half seconds, and so on, then it is obvious that 

 this sound must have come from exactly the spot 

 where the gun report originated. In plainer terms, 

 the hostile battery has suffered a direct hit. This, of 

 course, would be a rare occurrence with the first round. 

 Ordinarily the time intervals recorded for the burst 

 would differ somewhat from those of the German gun, 

 but knowing these differences, by means of a " gadget," 

 the computer could tell at once how far over or short 

 of the target, or how far right or left of it, the round fell. 

 The correction was sent to the gunner, and the British 

 gun continued firing and being corrected till the record 

 of the burst coincided with, or approached as nearly as 

 possible to, the original record of the hostile gun. 



This method of ranging was most conveniently used 

 when the usual kind of observation, whether by 

 aeroplane or ground observer, was impossible, as, for 

 instance, at night or at dawn, or on dull, foggy after- 

 noons. None the less it could be extremely accurate 

 work, and this accuracy could be tested by photo- 

 graphing from the air, subsequent to the shoot, the 

 neighbourhood of the battery fired on, and comparing 

 the positions of the shell craters with the points where 

 the sound-ranger said the rounds fell. 



On busy, noisy, and battle fronts, the work of the 

 sound-ranger was more difficult, partly because of the 



