302 Chas. Davison — British Earthquakes, 1891. 



seconds between them ; the intensity being nearly uniform through- 

 out, but greatest when the vibrations were felt, and slightly less 

 during the interval between them. Another observer at the same 

 place compares the sound to that made by an unusually heavy 

 waggon going over a bridge, but it was not heard by him during 

 the whole of the interval between the shocks. "There was silence," 

 he remarks, "for a second or two between the rumbles; the latter 

 went off as the sound of a heavy ground sea after breaking on. the 

 shore ; it was shorter and fainter than the first." At every other 

 place the sound ceased for a greater or less time between the 

 shocks. At Michaelstow a loud rumbling sound, resembling the 

 passing of a very heavy vehicle over a hard road, preceded each 

 shock, growing rapidly in volume and dying quickly away. At 

 St. Juliot, again, the sound accompanied each shock, varying only 

 slightly in intensity, but greatest at the moments when the vibrations 

 were felt. 



Origin of the Shocks. — The first poii^t to be determined is the 

 origin of the double shock, and of this three explanations may be 

 given. (1) The seismic focus may have consisted of two detached 

 portions, or have contained two regions of maximum initial intensity; 

 (2) there may have been a repetition of the originating impulse' 

 at one and the same spot ; or (3) a repetition of the originating 

 impulse at another and different spot ; the focus, or line, joining the 

 foci, being in any case directed north and south. 



If the first explanation wei'e the correct one, the interval between 

 the two shocks would have been greatest in the continuation of the 

 line joining the foci, i.e. in the north and south parts of the dis- 

 turbed area, and least in the east and west parts. Without placing 

 too great a reliance on the estimates of this interval given above, it 

 is clear that they offer little support to this theory. At Michaelstow, 

 indeed, the second shock was said to follow "almost instantaneously" 

 after the first, but the interval was yet long enough to allow the 

 rumbling sound to cease and to be heard again before the second 

 shock ; and Michaelstow is not far from the southern boundary 

 of the disturbed area. The second explanation is also out of the 

 question, on account of the different relative intensity of the shocks 

 at different places. 



There remains the third theory, and the evidence, such as it is, 

 seems clearly in its favoui*. At all parts of the disturbed area there 

 would be a perceptible interval between the shocks, provided the 

 interval between the initial impulses was not equal to the time 

 required for the earth-wave from a further and earlier focus to 

 overtake that from a nearer and later one. The distance between 

 the foci will, however, account to some extent for the variations in 

 the estimates of the interval between the shocks. 



More important evidence is furnished by the different relative 

 intensity of the two shocks at different places. We have seen that 

 the first shock was the stronger at Boscastle, Poundstock, and St. 

 Juliot, all lying to the north of the epicentrum ; the second decidedly 

 the stronger at Michaelstow, near the southern end of the disturbed 



