SOUND-RANGING 79 



smallest interval of which was one one hundredth of a second ; 

 the tenth second mark and the entire second mark were made 

 so as to be easily distinguishable from the others in order to 

 permit rapid counting. The photographic paper employed 

 was of the width of the standard moving picture film as this 

 could be obtained quickly and at low cost both in Europe and 

 America. 



Superimposed on the time scale on the paper were six 

 shadows evenly spaced across the width of the paper; these 

 shadows were cast by six tiny moving elements of a specially 

 constructed galvanometer. One of each of these elements was 

 electrically connected to a corresponding one of the listening 

 instruments of the sound-ranging base. When the sound of 

 a gun arrived at listener No. i there occurred a slight twitch 

 in the element No. I of the galvanometer and this twitch was 

 photographically recorded on the moving paper strip. The 

 time the twitch occurred could be read with an accuracy of at 

 least a hundredth of a second because the record of the twitch 

 and of the time were superposed on the same piece of photo- 

 graphic paper. When the sound arrived at listener No. 2 

 the element No. 2 responded and recorded the exact time as 

 just described for No. I and the same was true for the other 

 four elements. Thus if the mechanism were set in motion 

 before the sound of the gun reached the listener nearest to the 

 gun and was allowed to run until the sound reached the listener 

 furthest from the gun the photographic record which was de- 

 livered, automatically developed and fixed, contained all the in- 

 formation necessary to calculate the gun's position ; i. e., it con- 

 tained the five intervals between the times of arrival at the six 

 listening instruments. If, as generally, the recorder was run 

 for twenty or thirty seconds, the burst of the enemy's shell was 

 also recorded and its position could be reported to the artillery 

 in one to two minutes after the gun had fired. 



Figure 5 illustrates the type of record obtained from various 

 types of German guns variously located with reference to the 

 listening instrument. Figure 5 A shows the " twitches " on 



