Dr. C. Davison on Earthquake- Sounds. 39 



the epicentre. This certainly is the case, but the change is 

 not a marked one ; for, in the Hereford earthquake, of all 

 the observers who refer to the point, the percentages of those 

 who heard the sound increase to a maximum and then die 

 away are 57*3 for the central counties, 53'7 for the inner 

 ring, and 53*1 for the outer ring. The corresponding per- 

 centages for those who considered the sound of uniform 

 intensity throughout are 8 0, ll'l, and 12*2. 



The test employed, however, is not a very clear one, for at 

 all stations but those near the boundary of the sound-area 

 some change in the intensity would be perceptible. A better 

 test is furnished by the observed nature of the sound, by the 

 increasing frequency of reference to the wind-type as we 

 recede from the epicentre, and by the comparison of the 

 sound at great distances to distant thunder. 



Variation in Change of Character. — In the Hereford earth- 

 quake, the percentage of the total number of accounts in 

 which any change w r as recorded is 19*4 in the central counties, 

 10*7 in the inner ring, and 2*8 in the outer ring, of counties. 

 In the first district the change of type is generally due to the 

 perception of the crashing or grinding sound which accom- 

 panied the strongest vibrations ; in the others, the change, 

 when observed, is almost invariably from a sound like wind 

 to a more rumbling sound at the time of the shock, or vice 

 versa. In every earthquake I have studied, it is only near 

 the epicentre that the explosive reports or crashes in the 

 midst of the rumbling sound are heard. At a moderate 

 distance, the sound before and after the shock becomes 

 smoother, while the sound which accompanies the shock 

 retains to a certain extent its rougher and more rumbling 

 character. As we approach the boundary of the disturbed 

 area, the irregularities are still further smoothed away, and 

 the only sound heard is like the low, almost monotonous roll 

 of distant thunder. 



Variability in the Sound due to its Neighbourhood to the 

 Lower Limit of Audibility. 



Inaudibility to some Observers. — Attention has been drawn 

 in the preceding pages to the extraordinary depth of the 

 earthquake-sound, so far as it can be inferred from the 

 efforts that are made to describe it. But the most decisive 

 evidence of the close neighbourhood of the sound to the 

 lower limit of audibility is furnished by the fact that the same 

 vibrations are heard by some and not by others. One 

 observer will describe the sound as like the rumbling pro 





