Dr. C. Davison on Earthy uake- Sounds. 59 



only in degree and in the method in which we perceive them, 

 of one and the same phenomenon. 



Relative Position of Sound- Area and Disturbed Area. — 

 The excentricity of the sound-area with respect to the iso- 

 seismal lines is one of the most significant phenomena of earth- 

 quake-sounds. In weak earthquakes the sound is a more 

 prominent feature than the shock, and it is in these that the 

 excentricity is most easily detected ; but I believe that, with 

 a sufficiently careful study, it would be found to exist in 

 nearly all earthquakes. Thus, in the Leicester earthquake of 

 1893, which disturbed an area 58 miles long from W. 38° N. 

 to E. 38° S., and 46 miles broad, the sound-area and dis- 

 turbed area apparently coincided. Near the north-west end 

 of the longer axis, however, the shock was felt, but no sound 

 was heard, by observers at five places; while at seven places 

 towards the opposite end, the sound was heard without any 

 shock being sensible. It is thus clear that the isacoustic 

 lines, if they could have been drawn, would, relatively to the 

 isoseismals, be displaced towards the south-east. 



The sound-area of the Pembroke earthquake of 1893 was 

 bounded by a line similar in form and size to the isoseismal 4, 

 such that, if the latter curve were shifted about ten or twelve 

 miles in the direction of its longer axis, it would roughly 

 coincide with the former line. In the Neapolitan earthquake 

 of 1857, the records collected by Mallet in his great work 

 show that the boundary of the sound-area is similarly displaced 

 in the direction of the longer axis of the innermost isoseismal. 

 The peculiar form of the isacoustic lines of the Hereford 

 earthquake of 1896 and their aberration from the isoseismal 

 lines have been alluded to above. The observations of the 

 Edinburgh and Lancashire earthquakes of 1889 are somewhat 

 scanty, but they appear to indicate that the sound-areas 

 were shifted towards the lines of the earthquake-faults *. 



In the slight earthquakes of this country the excentricity 

 of the sound- area is manifested by its overlapping the dis- 

 turbed area in one direction. In the accompanying map (p. 60) 

 the continuous lines represent the isoseismals of intensities 3 

 and 4 of the Cornwall earthquake of April 1, 1898. The outer 

 dotted line indicates roughly the boundary of the sound-area, 

 and the inner dotted line, which is concentric with the former, 

 separates the places where the sound was very loud from 

 those where it was distinctly fainter. The relative positions 

 of the isoseismals show that the earthquake-fault must hade 

 towards the south-east, and therefore that the sound-area, 



* Geol. Mag-, vol. viii. 1891, pp.64, 311. 



