Dr. C. Davison on Earthquake- Sounds. 53 



audible than those which originate at a greater depth, and 

 that the curves of highest percentages correspond to the 

 earthquakes with the shallowest foci. It is therefore possible 

 that the axes of the systems of curves in fig. 2 may mark 

 out approximately the lines of growing faults. 



Relation between tJie Intensities of the Shock and Sound. 



Within and near the epicentral region of an earthquake, 

 the intensity of the sound increases and decreases with that 

 of the shock. Also, when the shock consists of two series of 

 vibrations or contains two maxima of intensity, the stronger 

 series or maximum is accompanied by the louder noise ; but 

 the difference of intensity in the case of the sound is much 

 less pronounced than in that of the shock. 



In different earthquakes, again, while the intensities of the 

 sound and shock certainly appear to increase together, the 

 sound which accompanies the stronger shocks is not much 

 louder than that which attends the weaker ones at places 

 near the epicentre. Similar terms are used in describing 

 both : in the one case, for instance, the rumbling of a train 

 going over a bridge only intensified a hundred times ; in the 

 other, the rumbling of thirty or forty waggons travelling at 

 a rapid pace. Humboldt also notices that " the intensity of 

 the hollow noise which generally accompanies an earthquake 

 does not increase in the same degree as the force of the oscil- 

 lations " *. In the Comrie earthquakes, according to one 

 observer, " a loud noise accompanies every shock, propor- 

 tionate in loudness to its strength, . . . but in the lesser ones 

 no vibration is perceptible, merely the peculiar report is 

 heard." Again, in October 1846 three great shocks occurred 

 in New Zealand, and were succeeded by numerous slight 

 tremors. " In the two last of the three grand shocks/'' 

 writes Sir W. Fitzherbert, " the roaring was certainly con- 

 siderable, but it was inconsiderable when compared with the 

 concussion, whilst in the minor shocks the vibration was 

 trifling as compared with the report " f. 



Relations between the Sound-Area and the Disturbed Area. 

 Relative Magnitude of Sound-Area and Disturbed Area.^- 

 The ratio which the extent of the sound-area bears to that of 

 the disturbed area ranges continuously between the widest 

 possible limits. At the one limit, the shock is felt but is 

 unaccompanied by sound ; at the other, the sound is heard 

 without any attendant shock. 



* Cosmos (Bohn's edition), vol. i. 1849, p. 203. 



f Quoted by Sir. J. Hector, Austral. Assoc. Trans, vol. iii. 1891, p. 523. 



