STUDIES IN NATURE 



surface of the ground, cannot move as fast as 

 the upper air, where there is nothing to get in 

 the way. Thus the top of a sound-wave moving 

 with the wind travels a little quicker than the 

 bottom ; the front of the wave as it moves will 

 lean forwards, and the sound will therefore con- 

 stantly tend to move downwards ; it will keep 

 near the surface of the ground, and be audible 

 to anyone on the ground at a great distance. 

 But now let us imagine that the sound is 

 moving against the wind ; the top of the wave 

 will now be kept back by the wind more than 

 the bottom ; the bottom of the sound-wave will 

 therefore move fastest, and the wave bends 

 backwards ; it leaves the surface of the ground, 

 and advances into the upper regions of the air. 

 To a person on the ground at a distance from 

 the source, the sound may be quite inaudible, 

 for most of the disturbance has gone over his 

 head. We can confirm this explanation by lis- 

 tening for a distant sound travelling against the 

 wind when we are on the top of a church tower, 

 or in some other high position. We shall find 

 that a distant sound which we can only just 

 hear from the ground is much louder at the 

 top of the tower. In fact, if the tower is high 

 enough, or the wind is not too violent, from the 

 top of the tower the sound is heard better 

 against the wind than with it. 



Besides the direction and strength of the 

 wind, many other conditions of the atmosphere 



(46) 



