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probably deeper), more rumbling and less crashing, than that of the 
Zeppelin bomb. 
In the inner sound-area, the sound is usually described as a roar or crash 
or heavy thud — such as the report of a 6-inch gun, very sharp and distinct, 
or a sudden, shattering, thunderous clap, very loud and short without any 
after-booming, similar, I suppose, to the sound of thunder as heard by a 
person walking along a noisy street. Of the detailed accounts (117 in 
number), reference is made to the bursting of Zeppelin bombs in 34 per 
cent., thunder in 25 per cent., the firing of a heavy gun in 24, and to 
miscellaneous sounds in 17 per cent. In about 10 per cent, of the records 
the detonation was followed by a rumbling noise, as, for instance, in 
comparison to a sharp detonation followed by a dull rumble, or a loud 
clap of thunder followed by a rumbling sound. The mean of 58 estimates 
of the duration of the sound is 2*8 seconds, but these estimates do not 
include the frequent records in which the sound is merely described as 
momentary. 
The outer sound-area, as will be seen in the next section, was a region of 
double- or multiple-reports. The detailed accounts are 173 in number, in 
75 per cent, of which the sound is compared to the explosion of Zeppelin 
bombs, in 15 per cent, to the firing of heavy guns, and in 10 per cent, 
to miscellaneous sounds. These proportions seem to point to the greater 
sharpness and brevity of the reports in the outer as compared with the 
inner sound-area. A rumbling or rolling sound was, however, heard occa- 
sionally, and almost equally by persons indoors or outside. 
Multiple-Reports. 
Speaking generally, the inner sound-area was a region of single-reports, 
and the outer sound-area one of multiple-reports. From the inner area 
there are 167 records of single-reports and 11 of multiple-reports. From the 
outer area there are 28 records of single-reports, 102 of double-reports, 74 of 
triple-reports, and 16 of quadruple-reports. Thus the percentage of single- 
reports from the inner area is 94, that of multiple-reports from the outer 
area is 77. 
The latter figure is, however, too small, for in some cases the records 
of single-reports in the outer area are clearly due to incomplete observation. 
From 32 places in this area I have received two or more records with 
regard to the number of reports heard. Single-reports were heard at 8 
of these places, and at every one of them two or more reports were heard by 
other observers. Again, at 17 other places double-reports were noticed, 
and at 9 of them triple- or quadruple-reports were also observed. 
