Dr. C. Davison on Earthquake- Sounds. 35 



anything else, — certainly much deeper toned and more awful," 

 while that which was henrd with the weaker and shorter 

 shocks was compared to " the noise caused by the blasting of 

 a soft rock at a considerable distance, followed by the rever- 

 beration of the surrounding mountains." The type of com- 

 parison employed therefore depends on the duration of the 

 sound, and this ao-;iin on the length of the seismic focus. 



Variation in the Nature of t lie Sound. 



Variation in Intensity. — Even if there were no definite 

 reports on this point, it would be evident, from the frequent 

 references to passing vehicles, that the sound is often of 

 varying intensity. Sometimes the sound becomes gradually 

 louder and ceases almost abruptly, or it may begin suddenly 

 and die away gradually; but in these cases the more or 

 less rapid termination is probably subjective and due to 

 the observer's inability to hear deep sounds. Tn each of 

 the three most important British earthquakes of the last 

 decade (namely, the Pembroke earthquakes of 1892 and 1893, 

 and the Hereford earthquake of 1896), more than 90 per cent, 

 of the observers who refer to the point distinguish some 

 change in the intensity of the sound ; while the percentages 

 of those who heard the sound become gradually louder and 

 then die away are respectively 68, 85, and 56. 



The relation which the change in intensity of the sound 

 bears to that of the shock is important. Generally, the two 

 increase and decrease together, though, as will be seen after- 

 wards, there are exceptions to this statement. Or, when the 

 shock is slight.it is felt when the sound is loudest, as at 

 Helston, during the Cornwall earthquake of May 17, 1892, 

 when " a distant rumble was heard, gradually increasing in 

 intensity until it became a violent thundering sound, at which 

 instant everything in the room was perceptibly shaken, the 

 sound dying away with the shock with the same sound as 

 before" *. 



Variation in Character. — As a general rule, the sound 

 adheres throughout to one of the types enumerated above, 

 and varies, if at all, only in intensity. In the neighbourhood 

 of the epicentre, however, there is also a change in character 

 perceptible to some, but not all, observers, the change taking 



* Mr. David Milne (Edin. New Phil. Journ. vol. xxxi. 1841, p. 261) 

 remarks that, in British earthquakes, there may be two perfectly distinct 

 sensations, a tremulous motion, and a violent blow or concussion. " The 

 tremulous or trembling motion," he says, " is always perceived. When 

 the blow occurs, it is generally in the midst of the tremors, and accom- 

 panied with the loudest noise." 



D2 



