68 OLDHAM: THE CACHAR EARTHQUAKE OF lOlH JANUARY 1869. 



about *7 and "25 of a mile respectively. Reducing the observations to 

 a common datum line, we obtain the following- depths of focus : — 



Yeddo— Max. 



352 miles 



Min. 



25-8 



Mean 



305 



from mean emergence 



303 



Teriaghat 42"7 

 There is, it will be observed, a considerable discrepancy between the 

 observations at Yeddo and Teriaghat; but the distance of the latter 

 place from the seismic vertical would cause any slight error of deter- 

 mination to be greatly magnified. The observations from Yeddo do not, 

 I am inclined to think, give too great a depth of focus, notwithstanding 

 that, if true, the depth in the case of this great earthquake exceeds that 

 of any other which has as yet been investigated. Yet when we consider 

 the vast extent of country over which it was felt, the great depth of 

 focus ceases to be astonishing ; for to effect this, not only is a high velo- 

 city of wave-particle required, but the focus must also be situated at a 

 considerable depth ; for the greater the velocity of wave-particle, the 

 further, cateris paribus, will the wave be propagated ; while if the depth 

 of focus be not great, the angle of emergence soon becomes nearly 

 horizontal, and so every inequality of the surface of the ground tends 

 rapidly to extinguish the shock. There are therefore good grounds for 

 declaring that in the case of this earthquake the mean depth of focus 

 was not less than 25 miles, but more probably about 30 miles, and may 

 have been as much as 35 miles. 



At Silchar, the evidence as to the angle of emergence is conflicting 

 and untrustworthy, — untrustworthy, as what it gives is not the angle of 

 emergence for that district, but merely an angle of emergence differing 

 from the true one on account of the peculiar situation of that town. 



The first evidence I shall produce is that given by the Police guard 

 a small two-roomed rectangular building shown in PL VIII. The walls of 

 this building bear N. 5°E., so that the shock was a subnormal one 

 and the angle of emergence, as given by the cracks, is evidently not far 

 from 45°. Now, as Silchar is more than 80 miles from the seismic verti^ 



( <3S ) 



