Miscellaneous Concluding Observations, 187
Determination of the Epicentrum.
Time Records. — If by means of accurate clocks scattered
about an earthquake-area the precise time of the shock could
be registered at a number of stations, it would be possible to
trace the disturbance to a station towards which all the time-
records would point as being the focus at which the earliest
movement occurred, or, in other words, the epicentrum. On
account of the great velocity of earthquake-movement it is,
however, evident that such records would have to be made
with extreme accuracy. Taking the mean radius of the
present earthquake as 135 miles (p. 22), and assuming from
Mallet's experiments that the velocity of transmission is
1000 feet per second (p. 38), the entire radius of 712,800 feet
would be traversed by the wave in nearly twelve minutes.
An examination into the most trustworthy of the time-
observations indicates, however, that the movement spread
with much greater velocity than this. If we take the velocity
in clay-rock as determined by Messrs. Milne and Gray,95 for
transverse vibrations we have the following results :—
Velocity = 254 x 103 centimetres per second = about 8000
to 9000 feet per second.
With this velocity the entire radius would be traversed
in about 11/4 minutes. In the present case we have no
trustworthy record from the extreme stations, the furthest
record being from Ashby Parva, about 100 miles from the
epicentrum, whilst the majority of observations to which any
weight can be attached are from stations much nearer the
centre of disturbance. The time-observations, to be of any
use in determining the epicentrum, should therefore have
been correct to within much less than 60 seconds, a degree of
accuracy to which no ordinary clock is kept. It appears,
indeed, that there is a greater divergence than this between
the records from Greenwich and Kew Observatories (pp. 150
He rejects as improbable the 1st, 6th, and 7th of these angles, and thus
limits the depth to from 11/2 to 5 miles. Prof. Milne adds:—This calcu-
lation for the depth of the shock, it will be observed, depends on a certain
vertical movement, which was registered. If this vertical motion is due
to transverse vibrations, the calculation is valueless.
95 Phil. Mag., Nov. 1881, p. 365.