Vol. 56.] CORNISH EARTHQUAKES OF 1898. 5 



curve is 0-8 mile N. 35° E. of the centre of the isoseismal 4, and the 

 distance between these two curves is 1*5 miles on the north-western 

 side and 0*8 mile on the south-eastern. It is thus evident that the 

 boundary of the double-shock area does not coincide with an isoseismal 

 line corresponding to any value between 4 and 3 : in other words, 

 that the foci of the two parts of the shock were not coincident. As 

 the area in question is that over which the weaker of the two parts 

 was felt, it corresponds therefore to the disturbed area of the first 

 part of the shock, while the isoseismal 3 bounds the disturbed area of 

 the second part. 



Sound-phenomena. — The sound was heard by 81 observers 

 at 51 different places, at 11 of which, however, no shock was felt. 

 From 5 places where the shock was felt I have no record of the 

 sound, but I think that the omission is accidental and due to the 

 brevity of the account. It is clear, at any rate, not only that 

 the sound-area included the whole of the disturbed area, but that 

 it even overlapped it towards the south-west, north, and north-east : 

 for the sound was heard by one observer at Kenneggy, by two at 

 Ealdhu, and by several persons at Tregothnau, the first two of 

 which localities lie about 1 mile and the third about 2 miles from 

 the boundary of the disturbed area. As the sound-vibrations in 

 these cases would come chiefly from the upper margin of the focus, 

 this overlapping confirms the inference that the hade of the fault 

 is to the south-east. 



Near the central part of the sound-area the sound was very loud, 

 and was a far more striking feature than the shock. At Mabe 

 one observer, who did not immediately recognize its seismic cha- 

 racter, remarked that it was the loudest thunder that she had ever 

 heard. Special reference to the loudness of the sound is made at 6 

 other places (namely, Bosvathick, Constautine, Gweek, Ponsanooth, 

 Trannack, and Trewardrevah) in such terms as ' very near thunder,' 

 ' a very heavy peal of thunder,' or ' a very loud explosion.' The 

 intensity of the sound, however, diminished very rapidly with the 

 increasing distance from the epicentre ; for at 14 other places, all 

 but two lying between the isoseismals 4 and 3, the sound is compared 

 to distant thunder, distant gun-firing, or to wind. The inner dotted 

 line on the map (p. 2) represents a line of equal sound-intensity, 

 separating the places where the sound was loud from those where it 

 was distinctly fainter. While it almost touches the isoseismal 4 on 

 the north-western side, it does not reach so far as this curve in the 

 opposite direction by nearly a mile, and it is thus approximately 

 concentric with the boundary of the double-shock area and also with 

 that of the sound-area. From the very rapid decline in the intensity 

 of the sound compared with that of the shock, we may infer that 

 the chief origin of the sound was not so deep as that of the more 

 prominent vibrations ; while the north-westerly shift of the line of 

 equal sound-intensity with reference to the isoseismal 4 implies again 

 that the hade of the originating fault is to the south-east. 



The sound was of the ordinary deep character, not much above 



