121 
1917 - 18 .] Sound-Waves of the East London Explosion. 
It may thus be concluded that, except perhaps for small terminal portions, 
the outer area was a triple-report area. 
Multiple- Reports in the Inner Area . — The 11 records of multiple- 
reports in the inner area may be divided into two classes : — (i) At 8 places 
within about a dozen miles of the source, a detonation, followed by one or 
two rumblings or minor explosions, was heard. (ii) At 3 places near 
the boundary of the area — Harpenden (28 miles), Stevenage (30 miles), 
and Horsell (30 miles) — there were two detonations in quick succession. 
The minor sound or sounds of the first class may have been merely rather 
prominent echoes, but the possibility of their pointing to secondary explo- 
sions must not be excluded. The sounds of the second class are, I believe, 
true multiple-reports. 
Multiple- Reports in the Outer Area . — It is evident that the multiple- 
reports of the outer area differed essentially from those heard over 
most of the inner area, for the first pair of sounds were almost equally loud, 
while the third and fourth were much fainter. It may be noticed here that 
the number of reports heard does not depend on the position of the observer, 
for multiple-reports are recorded by 87 per cent, of the observers indoors 
who stated their position, and by 86 per cent, of those outside. 
The map (fig. 2) shows the places at which single- or multiple-reports 
were heard. Single-report places are denoted by small circles, double- 
report places by dots, and triple- and quadruple-report places by crosses, 
the higher number being adopted when the observations from the same 
place differ as to the number of reports. It will be seen that double-reports 
were heard over by far the greater part of the outer sound-area, and triple- 
reports over a district but slightly less in extent. The latter were, indeed, 
absent only from the west end of the outer area, where the intensity of 
the sound was much less than elsewhere. 
With one exception (Peasenhall, near Saxmundham in Suffolk), 
quadruple-reports were confined to the county of Norfolk. They were 
heard at 11 other places, of which 8 lie within about seven miles of 
Norwich. Quadruple-reports were also observed at King’s Lynn, East 
Harling (near Thetford), and Redenhall (near Harleston), which lie in 
widely different parts of the county. 
It is difficult to speak with certainty as to the relative intensity 
of the reports. Some divergence of estimate is to be expected. When four 
reports were heard, however, there is no doubt that the first and second 
were much louder than the third and fourth, and that the last of all was 
weaker than the rest. The first pair were evidently nearly equal in 
strength, and they must usually have been the two reports heard over most 
