306 CHARLES DAVISON 
disturbed area, are conclusive against any theory based on the 
assumption of a single initial impulse. In the cases which have 
been investigated, there can be no doubt, I think, that there were 
two distinct foci, and that the impulses at them were nearly, but 
not quite, simultaneous. In the Charleston earthquake of 1886, 
Captain Dutton was able to locate the two foci; and this has 
also been done in several British earthquakes. 
There appear, however, to be two distinct classes of earth- 
quakes in which a double shock is observed; of which the 
Cornish earthquake of 1898 and the Hereford earthquake of 
1896 may be regarded as respective types. The chief outward 
difference consists in the length of the interval between the two 
parts of the shock. In the first case, the interval was a quarter 
of a minute or more in length; inthe second, it varied from a 
few seconds to zero. The Cornish earthquake consisted in real- 
_ ity of two successive earthquakes originating in nearly the same 
region of the fault, and the foci were overlapping. The Here- 
ford earthquake was a true fwzm earthquake, the foci were com- 
pletely detached; but the impulses at the two foci were due 
to the same initial stress, and the impulse at the second was 
in no way a consequence of that at the first, for it took place 
before the earth-wave from the first had time to reach the 
other. 
The two parts of a twin earthquake differ as a rule in intensity, 
in duration, in the period of their vibrations, and possibly in other 
ways. The distribution of the places where the first part was 
stronger, etc., than the second, enable us to determine at which 
focus the initial impulse was the more powerful and which was 
first in action. In the Hereford earthquake, the region in which 
the first part of the shock was stronger, of greater duration, and 
consisted of slower vibrations, was separated from that in which 
the same features characterized the second part, by a hyper- 
bolic band, passing between the two foci. Within this band the 
* The explanation of the double shock givenin the report on the Hereford Earth- 
quake (p. 295) I believe to be generally true for twin earthquakes; but I propose to 
consider the subject more fully in another paper. 
