Dr. C. Davison on Earthquake- Sounds. 43 



epicentre these changes in character must become gradually 

 less marked, until at a great distance, near the boundary of 

 the sound-area, the audible vibrations lie within very narrow 

 limits with regard to period, and the sound becomes an 

 almost monotonous deep growl of nearly uniform intensity. 



Isacoustic Lines. 



An isacoustic line may be defined as a line which passes 

 through all places in which the percentage of persons who 

 hear the sound is the same. 



The construction of isacoustic lines depends on the following 

 principles : — (1) at most places within the sound-area the 

 sound lies between the extreme values of the lower limit of 

 audibility, so that it is heard by some and not by others ; 

 (2) as the distance from the centre increases the audibility of 

 the sound-vibrations diminishes, so that the sound is heard 

 by a decreasing proportion of observers ; (3) the people in 

 any district possess various powers of hearing low sounds, 

 but in any one country the percentage of its inhabitants who 

 are capable of hearing a given low sound is probably in all 

 parts very nearly the same. 



Tn fig. 1 (p. 44) the continuous lines represent the isacoustic 

 lines of the Hereford earthquake of 1896. Their construction 

 depends on the percentage of observers within each county 

 who recorded their perception of the sound. The percentage 

 varies from 87 2 in Herefordshire to 23*1 in Essex, and for 

 any county it is supposed to correspond to that of a small 

 district immediately surrounding its centre, and for adjoining- 

 counties to vary uniformly from one centre to another. The 

 counties are of course too large and also too unequal in area 

 to allow of great accuracy in the construction of the lines, 

 and the number of observations in some of the counties at 

 a distance from the epicentre is not so great as is to be 

 desired *. 



* The inner isoseismal lines of this earthquake are elongated ovals 

 with their longer axes nearly north-west and south-east; and the 

 peculiar form of the isacoustic lines is due to the fact that there were 

 two distinct foci arranged along a north-west and south-east line, the 

 north-west focus (near Hereford) being first in action by a few seconds. 

 The series of vibrations from the two foci coalesced and formed one 

 series at places within a hyperbolic band, the curvilinear axis of which 

 coincides very nearly with the dotted line (tig. 1) passing through the 

 points of greatest extension of the isacoustic lines. The disturbance was 

 much stronger at the north-west than at the south-east focus, while the 

 inequality between the sound-vibrations from the two foci was incon- 

 siderable. There was therefore no marked distortion of the isoseismal 

 lines in the neighbourhood of the hyperbolic band, while the isacoustic 

 lines are completely diverted from their normal course. 



