286 Mr D. Milne on Earthquake- Shocks in Great Br i tain, 



transmitted across these faults.* Evidently they must be 

 transmitted more readily along parts which are unbroken and 

 compact than along parts that are shattered. They must, 

 therefore, be most extensively and strongly felt in directions 

 parallel with the faults and dykes. 



But this explanation does not meet all the conditions of the 

 question. It accounts sufficiently well for the direction in 

 which the vibrations are propagated, but it does not account 

 for the frequency of shocks in particular districts of country, 

 namely, those districts which are most shattered by dykes and 

 faults. If, as most geologists believe, earthquakes are caused 

 by the development of some mechanical power beneath the 

 earth's crust (whether exerted by elastic vapour or a heated 

 nucleus), it is evident, that this power will produce the great- 

 est effect on those parts which are the least able to resist it. 

 On that hypothesis, it is easy to understand how the upheav- 

 ing forces should obtain vent only or chiefly in those districts 

 which, in consequence of extensive dislocations, are capable of 

 most easily yielding ; and how the indications of these forces 

 should be most distinct along the lines of dislocations. The 

 same result must follow, on the hypothesis that earthquake- 

 shocks are caused by the relation which the state of the at- 

 mosphere bears to the state of the earth's nucleus ; and that 

 a communication between them is effected, by the fissures which 

 reach from the surface to the interior of the earth. These are 

 the two most probable hypotheses to account for the produc- 

 tion of earthquakes ; and in both of them, it is manifest how it 

 is that the shocks should be most frequent where deep and 

 extensive dislocations prevail. 



6. On glancing over the register, it will be found that the 

 shocks seldom occur single, and that there are generally tivo 

 in quick succession, as is well shewn under dates May 173G, 

 12th March 1795, September 1801, and August 181G. 



But farther, it deserves attention, as is well indicated by Mr 

 Gilfillan^s Comrie register, that the shocks come very fre- 

 quently in groups^ i. e., there w^as a succession of them for 

 many days, and sometimes for several weeks, the strongest or 



* In the notice of the shocks along the sea-coast of Hampshire in 1707, 

 it is specially stated, that it was "not felt at all to the north of the Downs/* 

 u e.f oB the north side of the fault which traverses that district. 



