SOUND-RANGING 



75 



a qualitative nature and may very seriously affect the operation 

 of a sound-ranging section, since corrections are usually not 

 possible. If the wind be blowing in a general direction towards 

 the enemy the sound of his guns may not arrive at the listening 

 instruments because of a lifting of the sound waves as they 



' Wind 



Figure 2 



advance against a wind whose velocity is generally greater at 

 the higher levels than it is near the ground. This case is 

 illustrated in Figure 2. 



If the direction of the wind is from the enemy guns the 

 sound reaches the listening instruments, but it is prolonged 



Ground Leve 



Figure 3 



by reason of echoes from the ground and the result of this 

 is that the time of arrival is not clearly marked. This 

 case is illustrated in Figure 3. Echoes also occur because of 

 temperature inequalities which cause reflection of the sound 



