76 OLDHAM: THE CACHAR EARTHQUAKE OP 10TH JANUARY 1869. 



the prism, but in addition it must be fractured from its base, for which 

 a velocity is required of 



2—2 L 5 v C0S & 



* -3 - 3 9 X a 3 X cog ^ + ^ 



whence 



v 3 = 9-9 



where L—^* Adding these tog-ether we obtain the result that a velo- 

 city of 17*8 feet per second would be required to fracture from its base 

 and overturn this tomb as a whole. 



But what is the velocity of wave -particle which will suffice to over- 

 throw the upper half of the tomb by itself ? Here there is no adhesion 

 to overcome, and the only resistance is that of inertia ; we may regard 

 this as that of a prism of 3 feet 3 inches in height and 1 foot 10 

 inches in breadth ; here a = 3"Zb, b = 1*8, = 29 ,0 8' — whence by the 

 same formula as before we obtain the result 



v x = 8" 6 feet per second. 



As the lower part of the tomb by itself would require for its over- 

 throw a far greater velocity of wave-particle than can be conceived as 

 possible, I shall not here enter into any detailed discussion of the subject. 

 These calculations show that if the velocity of wave-particle exceeded 

 8 \ feet per second, the upper portion of the tomb would be overthrown 

 by itself, and that in consequence of the break of continuity in its sub- 

 stance, it could not be overthrown as a whole except by a velocity 

 of wave-particle far greater than what we have any grounds for 

 regarding as possible. The aspect of the tomb after the passage of the 

 shock, as shown in pi. X, fig. %, exactly fits in with the former supposi- 

 tion; the lower portion of the tomb is still standing, only the upper has 

 been overthrown, and the slab marking the junction of the two portions 

 can be seen almost in its original position. This proves that the velocity 

 of wave-particle exceeded -8"6 feet per second. 



* L is the modulus of dynamic cohesion, or the length in feet of a column of material 

 whose weight, if suddenly applied, would produce a fracture in its substance by tension. 



( 76 ) 



