118 
Proceedings of the Eoyal Society of Edinburgh. [Sess. 
Rough though the scale may be, the results are remarkably concordant ; 
the places under each degree of the scale are closely grouped together, so 
that the lines which separate one group from another may be regarded as 
approximately lines of equal sound-intensity or isacoustic lines. These are 
indicated by the broken lines in fig. 1. In each area the boundary is an 
isacoustic line of intensity C ; the line of intensity B appears in both areas ; 
that of intensity A in the inner area alone. 
The curves show very clearly how much more rapidly the intensity of 
the sound diminished outwards in the inner sound-area than in the outer. 
In the inner area, the isacoustic line of intensity A includes an area of 104 
square miles, that of intensity B an area of 950 square miles. In the 
outer area, the isacoustic line of intensity B contains 2740 square miles, 
which is thus about three times as great as the corresponding area within 
the other curve. 
Though the places of observation are less closely grouped outside than 
within the isacoustic line of intensity B, it is remarkable how numerous 
they are near the lateral boundary of each area. There is none of that 
increasing rarity of observation which is so marked a feature near the 
boundary of the disturbed area of an earthquake. It would almost seem 
as if the sound failed to be audible outside the lateral boundaries, not 
because it was too faint to be heard, but rather because sound-rays were 
practically absent beyond the limits of those areas. At the ends of both 
areas, however, the sound was so weak that it was only heard by a few 
observers in each case. 
Sporadic Observations. 
In all great explosions, records of the sound come from places isolated 
from the well-defined sound-area or areas. In the present case the number 
of such places is small. 
In the silent zone, a noise like that of an explosion was heard in the 
highest part of Ipswich at about 7 p.m. Sound is also recorded at two 
places near the western end of the zone. At Uppingham a very distinct 
rumble, not unlike that of a distant Zeppelin bomb, was heard at about 
7.15; at Lilford (near Oundle) a boom was heard at 6.40. These two 
places, which are indicated by small crosses in fig. 1, lie in the narrowest and 
highest portion of the silent zone. The divergence of the estimated from 
the correct times casts some doubt on these two records, though the differ- 
ence is well within the error of many household clocks. 
A few records have also come from places beyond the outer boundaries 
of both sound-areas. Those from Kingsdown (near Deal) and Reading are 
