123 
1917-18.] Sound-Waves of the East London Explosion. 
of the outer area. When the sound is compared to the explosion of two 
Zeppelin bombs at a distance of five miles — and such observations are by 
no means uncommon — they cannot have differed much in loudness. If 
anything, the first may have been the stronger, for this order is given in ten 
records out of twelve of the double-sound. But here, again, comes in 
the uncertainty whether the first pair of reports were not grouped together 
in the observers’ minds as one. On the whole, I incline to the opinion that 
each report was weaker than the preceding, but that the difference in 
strength between the first and second and between the third and fourth 
was but small. 
It is clear, from the evidence, that the intervals between the reports 
were very short. In all parts of the outer area the first pair occurred in 
quick succession. The interval between the reports was probably about 
half a second. In not a few cases the second occurred before the first had 
ended, and this accounts, I think, for the records of single sounds. Between 
the second and third the interval was slightly longer, about one second ; 
while that between the third and fourth was again a fraction of a 
second. The three or four reports were thus grouped within an interval of 
two or three seconds. 
It is possible, as already mentioned, that the initial impulse was neither 
single nor instantaneous ; but it is certain that, if so, one of the explosions 
was far stronger than the others. The first and second reports heard in the 
outer area, being nearly equal in strength, must clearly have resulted 
from the sound-waves of the main explosion travelling by different paths. 
It is, of course, possible that the third and fourth resulted in like manner 
from a minor explosion quickly following the main one, but the evidence is 
insufficient to prove or disprove this explanation. 
Inaudible Am- Waves. 
On the accompanying map (fig. 3), the places at which windows 
rattled are indicated by crosses ; and those at which pheasants were 
disturbed, by dots. It will be seen that the dots predominate in the 
outer sound-area, which is a well-known pheasant-preserving country. 
In the inner area the records refer as a rule to the rattling of windows. 
The Shaking of Windows . — In some parts of the sound-areas, and 
especially in the western portion of the inner area, there was little if 
any wind at the time of the explosion. At many places in this district 
it was noticed that windows suddenly rattled with some violence, and 
then, after a few seconds, returned to rest. In some cases the shaking was 
repeated after a second or two. Thus, in the inner area, windows rattled 
