78 OLDHAM: THE CACHAR EARTHQUAKE OF 10TH JANUARY 1869. 



There remains one more index as yet not noticed. The boundary 

 wall of the' cemetery, which, though cracked, was not overthrown, will 

 give a maximum value for the velocity of wave particle. It may be 

 regarded as a rectangular prism affected by a subnormal shock, for the 

 direction of the wall is orthogonal to the direction of emergence of the 

 shock. Here = 18° %& a = 3 and b = 1. 



o a ,—s, s 1 COS 



1 6 * COS ■* (0 + e) 



v ± = 3*96 feet per second. 



This is the velocity for overthrow when fractured from its base. The 

 velocity for fracture taking L = 4 is 



_ „ Lb cos 



V = iff x— x 



a cos (0 + e) 

 v a = 14-17 

 Adding these together we obtain the total velocity required for fracture 

 and overthrow 



V = v x + «; a = 18-13 



Or about 3 feet less than that given by the cap of the gate pillar. As 

 there are not sufficient data for determining the proper value of L in this 

 case, I have purposely taken it low. "We are told that the mortar was 

 indifferent, but if the bricks had only a low specific gravity, this would 

 to a great extent counterbalance the want of quality and age in the 

 mortar work ; indeed, if we only take L = 5 feet instead of 4 feet, then 



Vc, = 17-89 

 and V = 21-85 



rather higher than that given by the cap of the gate pillar. 



As it is almost certain that in this case L could not have had a very 

 high value, it is evident that the wall was in a very critical state, and 

 that a slightly higher velocity of wave-particle would have sufficed for 

 its overthrow. 



( 78 ) 



