POPULAR SCIENCE 633 



inner area not far from that side from which the wind comes* 

 and the outer sound-area also lies on that side of the source 

 which is opposite to the direction of the wind. 



One other point is worthy of notice before leaving this dia- 

 gram. It is evident that many of the sound-rays (c) proceeding 

 with the wind enter the upper atmosphere and are lost to 

 observers on the ground. In the opposite direction, a large 

 proportion of the sound-rays (d, e) finally reach the outer area ; 

 and this perhaps accounts for the extraordinary intensity of 

 the sound which is developed within the outer area as compared 

 with that near the margins of the inner area. 



If the temperature of the air and its velocity at every point 

 were known, it would, of course, be possible to calculate exactly 

 the form of the lines shown in Fig. 5. But, as we are ignorant 



Plreotton of Wind 

 ■ ■' ■ ■ > 



L ' 



' , . N \ 



s » 





.>- 



Fig. 5. 



Diagram showing the course of the sound-rays when the velocity of the wind first increases and then 



decreases as the altitude increases. 



of these essential data, all that we can do is to make certain 

 assumptions, which, so far as we know, do not differ very widely 

 from the truth. Taking certain values which are of not un- 

 common occurrence for the rates of increase and subsequent 

 decrease of wind- velocity, and for the height at which the change 

 takes place, Mr. S. Fujiwhara, a Japanese mathematician, finds 

 that the outer boundary of the silent zone would lie about 

 seventy miles from the source of sound, a distance which does 

 not differ materially from that observed in some explosions. 



The atmospheric conditions are not always, perhaps not 

 often, so simple in character as those described above. Some- 

 times the wind remains nearly steady in direction and velocity 

 up to heights, it may be, of four or five miles or even more. Or 

 the velocity of the wind may decrease upwards ; and, in both 

 cases, we could not expect to find a silent zone developed. Fre- 

 quently, also, as may be seen by watching the movements of the 



