Dr. C Davison on Earthquake-Sounds . 37 



and decreases in intensity, the sound grows louder and dies 

 away, and also deepens and rises in pitch ; in other words, 

 the vibrations which are observed as shock and sound increase 

 and decrease together in amplitude and period. 



Variation in the Nature of the Sound throughout 

 the Disturbed Area. 



In one respect the sound exhibits a marked uniformity all 

 over the sound-area, namely, in its great depth, if we may 

 judge from the frequency with which the word " heavy " 

 occurs in the descriptions. In the Hereford earthquake, the 

 word is used in 27 per cent, of the comparisons from the 

 seven central counties, in 30 per cent, from the counties 

 surrounding these, and in 27 per cent, from those which lie 

 outside. In the Pembroke earthquake of 1892, the corre- 

 sponding percentages are 38, 45, and 31, and in that of 1893 

 they are 51, 37, and 39 *. 



Variation in Type. — The connexion between frequency of 

 type-reference and distance from the epicentre is shown 

 roughly in the Table (p. 38) . 



The principal change common to all three earthquakes is 

 thus the increasing percentage with the distance of the com- 

 parisons to wind ; and we may infer from this that the sound 

 tends to become smoother and more nearly monotonous as we 

 recede from the epicentre. As a rule, the comparisons to 

 passing waggons &c. are less numerous at a distance than 

 near the epicentre. The reasons for this appear to be that 

 the duration of the sound diminishes as the distance increases, 

 and that the rise and fall in intensity are also less marked. 

 In the Hereford earthquake it is possible to draw curves of 

 equal percentage of reference to this type, and these show 7 

 that the frequency of reference depends on the duration of 

 the sound. 



Variation in Cliange of Intensity. — One effect of distance 

 would naturally be to quench the weaker sound-vibrations at 

 the beginning and end, and we should expect to find less 

 variation in the intensity of the sound at a distance than near 



* In the Hereford earthquake, the central counties are those of 

 Hereford, Gloucester, Worcester, Shropshire, Radnor, Brecon, and 

 Monmouth ; the inner ring consists of Somerset, Wiltshire, Berkshire, 

 Oxford, Warwick, Birmingham, Stafford, Cheshire, Flint, Denbigh, 

 Montgomery, and Glamorgan ; and the outer ring of the remaining 

 counties in which the sound was heard. In the Pembroke earthquakes 

 the central counties are Pembroke, Carmarthen, and Cardigan ; those in 

 the inner ring are Cornwall, Devon, Glamorgan, Brecon, Radnor, 

 Montgomery, and Merioneth ; while the outer ring consists of the 

 remaining counties of the sound-area. 



