—— ay a 
Dr. C. Davison—British Earthquakes. 415 
sound-waves recede from the centre is distinctly marked. Within 
the central isoseismal, 97 per cent. of the observers on an average 
hear the sound, and in the successive zones bounded by the isoseismals, 
the average percentages of audibility are 94, 88, 69, and 60 respectively. 
Close to the boundary of the sound-area, there is a rapid decline 
to zero. 
The sound may be classified under one of the following types: 
(1) Wagons, trains, traction-engines, etc., passing, (2) thunder, 
(3) wind or a chimney on fire, (4) loads of stones falling, (5) the fall 
of a heavy body, (6) explosions, and (7) miscellaneous sounds, such 
as the trampling of many animals, the roar of a waterfall, etc. The 
following Table (IV) shows the average frequency of reference (in 
percentages of the total number) to these different types for the three 
classes of British earthquakes. In the last two lines of the table are 
given separate figures for two divisions of slight earthquakes, the 
first division haying a long focus and the second division a short 
focus. 
Taste LY. 
Type. 
Harthquakes. 
1 2 3 t 5) 6 7 
Strong . : ; . | 46 24 10 5 3 7 5 
Moderate . : F a, 29 26 4 5 5 8 3 
Slight : c c 5 393 29 5) 7 9 14 3 
,, (long focus) . - | 40 37 + 1 4 10 4 
,, (short focus) . 5 || 29 26 6 9 11 16 3 
Omitting the seventh type, the first three may be regarded as of 
long and the next three as of short duration. The percentage of 
reference to types of short duration is 16 for strong earthquakes, 
19 for moderate earthquakes, 31 for slight earthquakes, or 16 for 
slight earthquakes with a long focus and 37 for those with a short 
focus. It will be noticed that the sounds attending strong and 
moderate earthquakes, and also slight earthquakes with a long focus, 
are approximately of the same character. 
In the neighbourhood of the epicentre, the sound varies greatly in 
character and intensity. When the shock begins, the sound becomes 
deeper and more rumbling, and with the strongest vibrations deep 
booming explosive crashes are heard by those observers who possess 
a low limit of audibility. In the zone outside the central isoseismal 
the crashes are less frequently heard, but still the sound changes 
perceptibly while the shock is felt, becoming rough and grating; 
while, near the boundary of the sound-area, it is a low monotonous 
moan like the boom of very distant thunder. Table V shows the 
variation in type in strong earthquakes, beginning with the central 
zone A, and continuing with the zones B, C, D, E, bounded by 
successive pairs of isoseismals. 
