THE DEPTH OF AN EARTHQUAKE CENTRUM. 223 



been of the nature of sudden explosive reports accompanied 

 by rushing, rolling sounds. The form of the area in which 

 these sounds were heard was closely similar to that of 

 the first two isoseismals. Except in the central area of 

 great disturbance, no sound was heard to accompany the 

 shock. 



Those at the northern and southern extremity of the 

 sound area all described what they heard as a ' low, 

 grating, heavy, sighing rush, of twenty to sixty seconds' 

 duration.' Those in the middle and towards the east and 

 west boundaries of this area described a sound of the same 

 tone, but shorter and more abrupt, and accompanied with 

 more rumbling. 



The nature of the arguments w^hich were followed in 

 discussing the sound observations will be found in the 

 chapter relating to these phenomena. 



A portion of the argument which it is difficult to 

 follow relates to the maximum rate at which it can be 

 supposed possible for a fissure to be rent in rocks, which 

 rate depends on the density and elasticity of these rocks 

 and other constant factors. 



Next it was observed that the paths of the waves 

 drawn on the surface, although generally intersecting in 

 a point, did not do so absolutely, but along a line 

 passing through the main focus some 7J miles in length. 

 This, coupled with the observations of sounds, led to the 

 supposition that the centre of disturbance, considered 

 horizontally, originated along a curved line passing through 

 the chief focus and the various intersections of the wave 

 paths. 



The last phenomena brought forward to assist in the 

 solution of this interesting problem were a study of the 

 tremulous movements that preceded and followed the 

 shock, and their relation to the sound phenomena. 

 11 



