12 On the Recent Earth-Tremors, 



minutes between the neighbouring stations of Flinders and 

 Wilson's Promontory. Again, it is strange that Geelong and 

 Wilson s Promontory, 120 miles apart, should both note it at 

 the same moment, when between Sale and Bairnsdale on 

 the same co-seismic lines, there is a difference often minutes. 

 I have sought to reduce the minor errors of observation 

 by ascertaining the mean time of the record of each colony, 

 and this method gives more intelligible results. 



Table of Mean Time. 



Tasmania ... ... ... 9*38 



Victoria ... ... ... 9-38-81 



New South Wales ... ... 9-43 



From the first table we learn that the shock was felt 

 simultaneously at Gabo and on the east coast of Tasmania, 

 the time being 9*35. 



The wave passed from the coast of Tasmania to Launceston 

 (a distance of 55 miles) in two minutes ; from Launceston 

 to Circular Head (110 miles), in three minutes, reaching 

 the latter place at 9*40. 



The accepted method of determining the seismic centre is 

 to group together the localities whose records, synchronize. 

 These will be found to arrange themselves in curved lines, 

 and the whole series of these curves will form more or less 

 perfect concentric circles, within the innermost of which lies 

 the seat of disturbance. 



This method gives us Gabo and the east coast of Tasmania 

 as points on inner circle; time, 9"35. The next ring yielded 

 by the reports cuts Launceston, Hobart, Wilson's Promon- 

 tory, Geelong, Melbourne, the Buchan, and Omeo, at 

 9"37-9 - 38; and, still further from the centre, the wave, at 

 9*40, strikes Cressy and Circular Head in Tasmania, 

 Beechworth and Warragul in Victoria, and Albury and 

 Bega in New South Wales. The outermost ring seems to 

 be recorded at Bombala, N.S.W., at 950. 



A glance at these rings shows that they are asymmetric, 

 from which we judge that the earthquake wave travelled out- 

 wards at unequal speeds, and we have other evidence to show 

 that the speed of a tremor varies with the elasticity of 

 the medium. Mallet's experiments showed that the shock 

 caused by blasting travels through wet sand at the rate of 

 951 feet per second; through friable granite at 1283 feet per 



