POPULAR SCIENCE 



PHYSICS IN WARFARE 

 FINDING GUNS BY PHOTOGRAPHING SOUND 



By H. spencer TOY. B.Sc, A.Inst.P.. F.R.A.S. 



Senior Mathematical and Science Master, Queen's College, Taunton ; late 

 Computing Officer, Sound-Ranging Sections, R.E. 



Sound-ranging was invented during the Great War for the 

 purpose of locating hostile batteries. It will be shown in the 

 sequel how remarkable was the degree of accuracy attained. 



Other methods had previously been used for finding the 

 enemy's guns. These were often liable to a fairly large error, 

 but they continued to be employed right up to the end of the 

 war, often as a check on the positions given by the sound- 

 rangers. 



Of these methods, two call for special mention, viz., " Flash 

 Spotting " and Aeroplane Photography. The " Flash Spotting " 

 was carried out by observation groups, special branches of 

 the Royal Engineers formed for this purpose, numbered con- 

 secutively from one upwards. The principle underlying their 

 work is that if a flash be observed from two or more stations, 

 and the corresponding bearings be noted, then, when these 

 bearings are plotted on the map, they intersect in a point which 

 gives the location of the gun. 



In practice, however, it was found that these lines intersected 

 at an extremely acute angle, so that the position of the gun 

 could be determined with but a fair degree of certainty. The 

 reason for this is that the stations from which observations 

 were taken were always fairly close together when compared 

 with the distance of any of them from the gun, which was 

 situated well behind the enemy's lines. 



In theory, aeroplane photography should give the position 

 of a gun with the utmost degree of certainty. In practice, 

 however, the objects appeared so small, even when viewed 

 through a strong magnifying-glass, and were so well camou- 

 flaged, that it was often impossible to say positively that the 

 given object was a battery. But if it was known by the sound- 



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