and the Conditions ivhich they Indicate. 15 



notable. Mr. Quail, the pilot, tells rne that nothing of the 

 kind was noticed there by him, although he was on the 

 beach when he felt the shock, and simultaneously saw 

 the sea agitated in an unusual manner. 



Thirdly, it may be explained by supposing that the 

 disturbed area was so close to the coast that the second 

 wave must have arrived as soon as the first, or so quickly 

 after it as not to have been distinguishable from it. The 

 secondary sea wave of an earthquake travels more slowly 

 than the first. On the occasion of the Lisbon earthquake, 

 the first wave reached Madeira in 25 minutes, but the 

 second took 2J hours, the rates of speed being as 6 is to 1.. 

 If the seismic centre of the recent shock was 120 miles 

 distant from the coast, and the speed of the tremor was 30 

 miles per minute, the secondary wave would reach the shore 

 20 minutes after the first, and this ought to have been noted 

 somewhere. But if the distance off shore was only 30 miles, 

 the secondary wave would arrive within six minutes after 

 the first. 



The conclusion I come to is, that the edge of the seismic 

 centre was close to the east coast of Tasmania and to the 

 south-east extremity of Australia. 



The last form of manifestation to be considered is the air 

 wave, which reveals itself by loud rolling rumbling sounds, 

 as of heavy-laden waggons. Such reverberations, loud and 

 long-continued, were noticed in innumerable places. 



I have now set out all the facts known to me concerning 

 the recent earthquake, and I have founded certain conclu- 

 sions upon them. A comparison of times and places and 

 directions of the shock indicates that the seismic centre 

 must have been about the point intersected by 150 E. long. 

 and 40 S. lat. The high angle of emergence at the Sand- 

 hills, Tasmania, where boulders were ejected from their beds ; 

 the high speed of the land tremor, and the absence of a 

 second sea wave, all point to the margin of the disturbed 

 area having been much nearer to the land than the focus. 

 The high speed of the tremor again points unmistakably to 

 a deep focus. For it is known that the speed of the wave 

 of elastic compression is greatest when it traverses rigid 

 strata, and that it slackens in decomposed, faulted, or friable 

 rocks. The wave could have attained to the high rate 

 reported locally only where are extensive masses of unbroken, 

 homogeneous, crystalline rocks. To obtain a free course 

 through strata of this character the wave must have 



