PHYSICS IN WARFARE 



93 



in front of it of a specially constructed cog-wheel, controlled 

 by a tuning-fork. This piece of apparatus will be considered 

 in greater detail later. 



The horizontal lines correspond to the six recording stations 

 in the field, each of the strings thus photographed being con- 

 nected to a microphone. The time differences, therefore, in 

 the sound reaching the stations are simply read off from the 

 film. In Fig. i this difference for the first two stations is about 

 twenty-four hundredths of a second, and for the second and 

 third about sixteen. 



These differences are then corrected to allow for the velocity 

 and direction of the wind, and for the temperature at the time, 

 both of which affect the actual velocity of sound. They are 

 then transferred to a map of the district, around the edge of 

 which a specially constructed scale is drawn. In practice, the 

 guns that had to be located were so far away, compared with 

 the distance between the microphones, that it was often found 

 sufficiently accurate to plot the asymptote of the hyperbola 

 instead of the curve itself, and even when this was not the case, 

 a suitable correction from previously calculated tables could 

 be applied. This made it possible to work with a straight 

 line instead of with a curve, which would have been trouble- 

 some to construct. The line was actually laid down on the 

 map by means of a fine thread which was pivoted at a point 

 midway between those marking the positions of the two micro- 

 phones in question and which was placed on a suitable reading 

 on the scale round the edge. The four other strings were 

 similarly placed, and the point at which they intersected gave 

 the location of the gun. 



We must now consider the 



mechanical means adopted to 

 make this photographic record of 

 time differences that we have been 

 discussing — means that illustrate 

 the resources of modern physics. 



Let A, B, C, D, E, and F 

 (Fig. 2) be the six observing 

 stations to which reference has 

 already been made, at each of 

 which a microphone is placed. 

 These stations were situated just 

 behind the front line, and were 

 frequently arranged on the arc of 

 a circle for convenience, although 

 this condition is not essential. Each station was connected 

 electrically to the headquarters of the sound-ranging section, 

 marked H.Q. in the figure, and situated about 3,000 to 4,000 



Our Own Front Line 



