POPULAR SCIENCE 631 



from the source, the intensity of the sound heard within that 

 area is extraordinary. At 84 miles from Spithead, the reports 

 of the minute-guns in 1901 were so loud that men labouring in 

 the fields stopped their work to listen. In the East London 

 explosion, the sound at no part of the outer area was so loud as 

 in the immediate neighbourhood of the ruined factory ; but 

 the area of moderate loudness within it was nearly three times 

 as great as that within the inner sound-area. 



Origin of the Silent Zone 



If the sound-waves were to travel continuously along or 

 close to the ground, they would be audible over the whole 

 district to the farthest point reached by them, and there would 

 be no silent zone. If they were to follow a course not far above 

 the ground, secondary waves w r ould spread downwards from 

 their lower edges, and would be heard here and there by persons 

 who are capable of hearing low sounds of slight intensity. 

 Thus, the mere existence of the silent zone is evidence that the 

 sound-waves cross it by an elevated route ; and, to discover the 

 origin of the silent zone, we have to ascertain the cause or causes 

 which lead the sound-waves to desert the neighbourhood of the 

 ground at the boundary of the inner area, and afterwards return 

 in but slightly diminished strength to the outer sound-area. 



It is well known that, in the neighbourhood of the ground, 

 the velocity of the wind increases with the height. If a sound- 

 wave were travelling along the ground in any direction inclined 

 to the horizon, the effect of this increased velocity would be to 

 tilt the upper part of the wave forward if it were advancing with 

 the wind, and backward if against it. As the wave travels in 

 a direction at right angles to its surface, it follows that the sound- 

 waves are tilted upwards if moving against the wind, and soon 

 pass over the heads of observers on the ground. In the opposite 

 direction, waves at first proceeding on an upward course are 

 deflected downwards, and are therefore heard along the ground 

 to a much greater distance than in the opposite direction. 



The diagram (Fig. 4) is designed to represent approximately 

 the paths of the sound-waves. The dotted lines are supposed 

 to be everywhere at right angles to the surfaces of the waves, 

 and therefore to represent sound-rays indicating at any point 

 the direction in which a wave is moving. They are drawn with- 

 out regard to scale or exact form. The object of the diagram is 

 simply to represent the facts that, if the velocity of the wind 

 increases upward, the sound-rays proceeding with the wind are 

 concave, and those against it convex, towards the ground. In 

 the line below the diagram, o represents the source of sound, 

 oa the range of the sound with the wind, ob the range against it. 



