extraordinary Agitations of the Sea. 51 



near Reading as the only places where shocks were felt in this 

 island; but there is abundant traditional evidence of shocks 

 having been felt in Cornwall ; whilst in the Scilly Isles " several 

 persons ran out of their houses for fear they would fall upon 

 them"*. 



On the other hand, if the centre of an earthquake be inland 

 and close beneath the surface, the shock would not travel verti- 

 cally or upwards at all, but horizontally, as the shock in this 

 country of the 6th of October, 1863, appears to have done ; for 

 when at Pontypool, in the midst of Welsh mines, soon after its 

 occurrence, I was informed by a gentleman who had ascertained 

 the fact from the miners themselves, that whilst the shock was 

 felt at the mines above ground, it was not felt by any under 

 ground. Nor does it appear to have been felt under ground in 

 any of the mines of Cornwallf. 



The two preceding paragraphs may help to account for a stri- 

 king fact mentioned by Mr. Mallet, viz. that the extraordinary 

 disturbances of the sea now under consideration have " never 

 been observed to take place in any earthquake whose centre of 

 impulse was inland, however violent " J. This fact (which is in 

 perfect agreement with my hypothesis) may perhaps be owing 

 to the centre of impulse, when inland, being generally close under 

 the surface, so that the shocks must travel horizontally, or nearly 

 so ; and it is not to horizontal, but to vertical shocks that I 

 have always ascribed these disturbances of the sea. 



Another fact not less remarkable is, that, while earthquakes in 

 general take place equally in all states of the atmosphere, those 

 which are known only by the extraordinary agitations of the sea 

 or lakes which they produce occur almost exclusively during 

 storms, or at or near considerable minima of the barometer. Is it 

 because these submarine shocks, as already stated, are almost 

 always vertical, or nearly so, while those on dry land are generally 

 horizontal? In vertical shocks there may be electrical discharges 

 between the earth and the atmosphere which might occasion the 

 attendant minima of the barometer, whereas in horizontal shocks 

 the discharges (if any) may be only between differently charged 

 portions of the earth without much affecting the atmosphere. 

 The first earthquake felt by Humboldt in Cumana was during a 

 severe thunder-storm. " At the moment of the strongest elec- 

 trical explosion were two considerable shocks of an earthquake f 

 but the barometer, which had been previously falling, continued 

 to fall for five hours afterwards, when a third and last shock 



* Troutbeck's < Scilly/ 1794, p. 40. 



t Journal of the Royal Institution of Cornwall, No. 1. p. 61. 

 X British Association Reports, vol. xx. p. 46. 

 E2 



