384 DR. CHARLES DAVISON ON THE fAug. 1902, 
Thus, as a general rule, the beginning of the sound preceded that 
of the shock, their epochs of maximum intensity coincided, while 
the end of the sound followed that of the shock. The most striking 
exception occurs in the case of Aberdeenshire, where, in the majority 
of cases, the three epochs coincided one with the other. This result 
is important in its bearing on the origin of the sound-vibrations, 
for most of the observations come from the south of the county, and 
the line joining this district to the epicentre is nearly perpendicular 
to the line of fault. Now, if the general precedence of the sound 
with respect to the shock were due to its superior velocity, the 
percentage of records in which the beginning of the sound preceded 
that of the shock would vary only with the distance, and not with 
the direction, from the origin. If, however, the sound-vibrations 
were to start simultaneously, or nearly so, from all parts of the focus, 
but especially from its marginal regions, then the three epochs of the 
sound and shock would coincide approximately at places near a line 
at right angles to the earthquake-fault. We may infer from the 
observed coincidence, and also from the fact that the sound generally 
followed the shock at distant stations, that the vibrations of every 
amplitude and period travelled outward with the same, or very 
nearly the same, velocity. 
The time-relations of both terminal epochs are recorded by 180 
observers, and from these we obtain the following table, in which 
are given the percentages of cases wherein the duration of the sound 
was greater than, equal to, or less than, that of the shock. ‘The last 
line contains the corresponding average figures for the Pembroke 
earthquakes of 1892 and 1893, and the Hereford earthquake of 
1896. 
County. | Greater. | Equan. | Luss. | Dovsrrut. 
HIVeTMGsSim. 2.4 saekes od eee 7d 10 | 1 14 
LoTR A ee ae ae ee ee 67 11 8 14 
eA DOMOCEI cre ssnanes eannae et 6 75 | 19 
| Whole sound-area ......... 60 14 | 3 23 
| Average for strong shocks . D7 18 | 5 20 
Omitting doubtful records, we find that, in 78 per cent. of the 
cases, the duration of the sound was greater than, in 18 equal to, 
and in 4 per cent. less than, that of the shock. It is possible that 
the movements which produced the sound-vibrations lasted a longer 
time than those which produced the shock. But the more probable 
explanation is, that the area from which the sound-vibrations 
proceeded*was larger than that whence the more prominent vibra- 
tions came, and extended beyond it at both ends; in other words, 
that the sounds heard before and after the shock came from the 
lateral margins of the focus. 
