PHYSICS IN WARFARE 91 



ranging results, or otherwise, that a battery existed in a given 

 small area, then it was often possible to say definitely that it 

 was situated at a particular point. 



To ensure a much greater degree of accuracy in these loca- 

 tions, sound-ranging was invented. The French commenced 

 experiments in October 19 14. The British took it up, and 

 at the end of 191 5 established an experimental section at 

 Kemmel Hill, just south of the Ypres salient. This proved 

 so successful that other sections were formed, and in 191 6 

 two of them were attached to each of the British Armies on 

 the Western Front. Their value exceeded all expectations ; 

 other sections were formed, and in 191 7 and 191 8 there were 

 more than thirty on the Western Front and about half a dozen 

 in Palestine and Salonika. 



The whole system of sound-ranging is built up on the simple 

 fact that sound travels comparatively slowly. Its speed is 

 really very great compared with the ordinary standards of 

 daily life, and is about 1,120 feet per second at ordinary temper- 

 atures ; but when compared with the speed of light, which is 

 186,330 miles per second, it is almost inappreciable. The 

 time taken by sound to travel some distance from the source 

 can be measured fairly accurately. That it does take some 

 measurable time is proved by many common daily observations, 

 such as, for instance, hearing an engine whistle after the escape 

 of steam is seen. If, therefore, the time taken by sound to 

 travel from its source to some observing stations be noted, the 

 position of the source can be calculated. 



In warfare, it is not possible to make such observations 

 accurately, but accurate observations can be made of the 

 differences in the times taken by sound to reach the various 

 stations, and on these differences the system is based. 



To appreciate the principle fully, let us consider for the 

 moment the beautiful curve known as the ellipse. If E and 

 F be the foci and P any point whatever on the curve, then 

 it is known that the sum of the distances EP and FP is a 

 constant. This, indeed, is the property made use of in 

 drawing the ellipse by the common method of two pins and a 

 piece of string. 



The hyperbola, on the other hand, has the property that 

 the difference of the distances EP and FP is a constant. 

 It follows, therefore, that if the constant difference be known, 

 a series of points P can be found, and so the hyperbola con- 

 structed. This fact is fundamental in sound-ranging. A gun 

 is fired from some unknown point behind the enemy's lines. 

 The times at which its report reaches two observing stations, 

 E and F, are noted, and so the difference is calculated. This 

 enables us to say with certainty that the gun must lie at some 



