Charles Davison—British Earthquakes. 301 
area, show that the length of the seismic focus must be reckoned in 
miles. ‘This is all that can be said, for estimates of duration are 
generally too great, and we are ignorant of the velocity of the 
earth-wave in the case of a slight earthquake. Also the process 
of slipping may have occupied an appreciable time. But the two 
classes of evidence do not conflict. For a distance, therefore, of 
about 24 miles on either side of the epicentrum B, the slip may have 
been comparatively great; and then it must have gradually died 
away in either direction until it became imperceptible. ‘These 
lateral margins of the focus cannot have been much under a mile 
in horizontal length, and they were probably more, especially 
towards the west. 
On this point, some important evidence is given by the obser- 
vations made at Penryn, Budock, St. Ives and other places, especially 
those near the longer axis of the disturbed area. Though the pre- 
cursory sound was observed at most places, it is clear that at those 
near the east end of the area the tremors and sound were chiefly 
perceived, or perceived for a longer time, after the shock ; and before 
the shock at places near the west end. This shows that the slip 
died out more gradually towards the west than towards the east. 
It is curious that records of the double vibration, or rather double 
character, of the second shock, come from only four places, namely, 
Redruth, Perran-ar-Worthal, Penryn and Breage. I believe, how- 
ever, that the absence of other records is accidental, for few observers 
perceive or record changes in the intensity. Now a double shock 
may be due (1) to the focus consisting of two detached, or practically 
detached, portions ; (2) possibly to the partial reflexion of the earth- 
wave; or (8) to a repetition of the original impulse. The dis- 
tribution of the four places mentioned above favours the latter 
interpretation, the first vibration being slightly more intense than 
the second. It is clear, too, from the observations recorded at St. 
Keverne and Phillack, that the amount of slip varied somewhat 
irregularly over the focus, giving rise to several maxima of sound- 
intensity, the feebler sound between the maxima not being per- 
ceptible to the observers at these places. 
Lastly, the simple character of the shock renders observations on 
the first direction of the vertical motion more reliable than usual. 
At Redruth, on the upthrow side of the fault, the movement is said 
to have been first downward and then upward. At Manaccan, on 
the downthrow side, it was first upward and then downward, and 
this, I am informed by the Rev. A. R. Eager, D.D., is attested by the 
evidence of several observers. This shows that the slip of the down- 
throw side must have been relatively downwards, thus increasing 
the throw of the fanlt. 
We may now sum up as follows. For some time the strain along 
this fault had been increasing. Hither, however, the strain or the 
resistance to it was not equally distributed, and first a slight slip 
took place at 10°30 p.m. on May 16. ‘This had the effect of 
equalising the strain or the resistance to it, and, consequently, three 
hours later, a second and double slip took place over a much greater 
