632 



SCIENCE PROGRESS 



As every part of the region from a to b receives sound-rays, 

 it is clear that, under such a condition of wind, there can be no 

 silent zone. 



If, on the other hand, the velocity of the wind were to de- 

 crease upward, the sound-rays would be curved in the opposite 

 directions. Those with the wind would be convex, and those 

 against it concave, towards the ground. The diagram (Fig. 4) 

 will represent the corresponding sound-rays, except that the 

 arrow showing the direction of the wind should be reversed, 

 while ob will represent the range of sound with the wind, and 

 oa the range against it. There would, again, be no silent zone. 



Now, let us suppose that the velocity of the wind increases 

 with the height up to a certain level, and then continually 

 decreases, there being no change in the direction of the wind. 



\ \ \ \ 





Direction of Wind 



■ " ■ - > 



1 ,- ^,- 



I / / y -- 



\ \ </ ^' _ 



Ji»_ 



BO A 



Fig. 4. 



Diagram showing the course of the sound-rays when the velocity of the wind increases with the altitude. 



Below the level ll at which the change takes place, the form 

 of the sound-rays would be as in Fig. 4, the rays a (Fig. 5) being 

 those with the wind, and the rays b those against it. Above 

 the level ll, the form of the rays would be, as in Fig. 4, reversed, 

 the rays c with the wind would become convex, and those d 

 against it concave, towards the ground. The former (c), it will 

 be seen, rise upward and are lost in the upper air. The latter 

 (d) return to the level ll, and, entering the lower atmosphere 

 (e), become convex again towards the ground. Thus the course 

 of the sound-rays would be roughly as shown in Fig. 5. In the 

 line below the diagram, o represents, as before, the source of 

 sound, oa the range of sound in the inner area with the wind, 

 and ob the range against it, bc the width of the silent zone, and 

 cd that of the outer sound-area. The corresponding map of 

 the sound-areas would resemble very closely that shown in 

 Fig. 3 ; for the source of sound occupies a place within the 



