125 
1917-18.] Sound-Waves of the East London Explosion. 
twice at Gravesend and Dorking ; and, in the outer area, twice at Boston, 
Swineshead (Boston), Arminghall (Norwich), Hempstead (Holt), Ludham, 
and King’s Lynn ; two or three times at Little Ponton (Grantham) ; and 
three times at Norwich, Great Yarmouth, and Lowestoft. 
Now, the sound- vibrations alone, acting on so large a surface, are 
probably capable of shaking loosely fitting windows, and it does not follow 
that every such shaking should be attributed to inaudible air-waves, unless 
it occurred before or after the instant at which the sound was heard 
or between or beyond the two sound-areas. 
In the inner area, windows rattled after the sound was heard at Pimlico 
(London), Barnes (12 miles), and East Acton (13 miles) ; and before at Luton 
(32J miles). Doors were shaken before the arrival of the sound-waves 
at Harpenden (28 miles). At Upper Norwood (11J miles) a window was 
blown outwards before the roar of the explosion was heard. In the outer 
area, windows were shaken before the sound-waves arrived at Great 
Ponton (Grantham, 99f miles) and Halton Holgate (Spilsby, 115 miles). 
The evidence on this point is unfortunately scanty. So far as it goes, 
it would seem to show that, so long as the inaudible waves and the 
sound-waves travelled together near the ground, the latter slightly outran 
the former ; but that, at considerable distances, the mean horizontal velocity 
of the air- waves was slightly greater than that of the sound-waves. 
Disturbance of Pheasants . — Pheasants are often disturbed by gun-firing 
or explosions when the sound is inaudible to man, and this is usually 
attributed to their keener powers of hearing low sounds. But, if we may 
judge from their behaviour during earthquakes, their powers in this respect 
are inferior to those of human beings, if, indeed, they are capable of hearing 
at all the sound of a great explosion. It is more probable that pheasants 
are alarmed by the sudden quivering of the trees on which they happen to 
be resting. 
As will be seen from fig. 3, pheasants were disturbed during the 
recent explosion over a district which includes not only the two sound - 
areas, but also the whole or a large part of the silent zone, and extends 
some distance beyond the farthest limits of the sound-areas. I have 
received altogether records from 153 places, of which 116 are in the outer 
sound-area (63 in the county of Norfolk alone), 18 in the inner sound-area 5 
while 19 lie in the regions between or beyond them. 
The behaviour of the pheasants was similar to that which they 
show when disturbed by thunder or heavy gun-firing or the explosion of 
Zeppelin bombs. The birds flew suddenly down from the trees on which 
they were roosting, or fluttered clumsily and restlessly from one tree 
