78 THE NEW WORLD OF SCIENCE 



photographic chronograph at a " central " or calculating sta- 

 tion so situated as to entail the minimum amount of wire con- 

 nection to the listening instruments. Each section had two 

 advanced posts at which observers were on duty day and night 

 in order to start the automatic recording mechanism at the 

 " central " when necessary. The listening instruments were 

 equi-spaced on a straight line, or more generally on an arc of 

 a circle which was concave toward the enemy, situated a short 

 distance behind the line of the advanced posts mentioned above. 

 The distance between the listening instruments was generally 

 about fifteen hundred meters so that the entire length of the 

 sound-ranging base was about seventy-five hundred meters or 

 slightly less than five miles. The employment of a regular 

 base, generally an arc of a circle, was a highly important in- 

 novation which was introduced by the British ; it rendered the 

 interpretation of the records easy even when there was con- 

 siderable artillery activity because the indications on the record 

 which were caused by any one of the many guns which might 

 be firing at about the same time were spread out on the record 

 in a simple geometric pattern if the listening instruments were 

 arranged on the ground in a simple curve. Owing to this it 

 was possible to locate several guns, firing practically simul- 

 taneously, without a loss of time in correctly interpreting the 

 photographic record delivered by the instrument at the central 

 station. 



The recording mechanism at the " central " consisted of an 

 accurate timing device arranged so as to photograph on a 

 moving strip of sensitized paper a series of lines about one- 

 fiftieth of an inch apart ; these lines were the shadows cast by 

 a set of spokes of a wheel which was kept spinning in the path 

 of a beam of light which fell on the sensitized paper; the rate 

 of spin of the spoked wheel was governed by a tuning fork so 

 that the shadows were cast on the paper with the greatest at- 

 tainable regularity; the rate chosen was one hundred shadows 

 per second so that the photographic paper had recorded on it 

 across its entire width an extremely accurate time scale the 



