SOUND-RANGING 71 



speed, to the nearest whole number, is 337 meters per second. 

 The velocity is not only affected by the temperature of the air 

 but also the apparent velocity is very markedly affected by the 

 velocity and direction of the wind. It follows from this that 

 a survey carried out by the means of sound waves, unlike a 

 survey carried out in the ordinary manner by -light waves, is 

 subject to errors introduced by the lack of accurate knowledge 

 of the wind and temperature corrections which it is necessary 

 to apply to the data of observation. Furthermore there is a 

 lack of parallelism between a light survey and a sound survey 

 which will be evident from the following consideration. To 

 locate a point on the ground by a light survey it is only neces- 

 sary to secure an intersection of two light beams from two 

 known points on a surveyed base line by the use of relatively 

 small telescopes, while to obtain a location at all comparable in 

 accuracy by means of sound it would be necessary to use in- 

 struments of prohibitively great size. Fortunately, however, 

 advantage may be taken of the low velocity of sound compared 

 to that of light to obtain a survey from three points without 

 the use of listening apparatus of great size. This method en- 

 tails the accurate measurement of the differences of times of 

 arrival of sound at the three points. This, of course, requires 

 the use of some form of accurate clock and precludes the use of 

 human observers who are likely to differ so much in their reac- 

 tion times that their results are only roughly comparable. 



In still air at 10 degrees centigrade the sound from a gun 

 moves out in a wave of compression and rarefaction which 

 travels 337 meters per second. If the gun is at G and 

 mechanical listeners electrically connected to a common timing 



1 The Germans employed a sound-ranging system with human ob- 

 servers especially trained and selected to have equal reaction time but 

 the results obtained by the Germans fell far short of what the Allies 

 accomplished. The German system was defective not only as regards 

 accuracy but also as regards the speed with which results could be 

 reported; what the Allies could do in two minutes took the Germans 

 nearly an hour. 



