OP THE VELOCITY OF MOTION OF THE WAVE- PARTI CLE. 75 



marked a in PI. X, fig. 1, and can be seen lying almost in its original 

 position in PL X, fig. 2. 



The tomb may then be considered as being divided into two portions, 

 one above and the other below this marble slab ; we may consequently 

 regard it as two solid rectangular prisms superimposed one on the other, 

 and, practically, without any cohesion between them. A body of this 

 description may be overthrown in either of two ways : the whole may 

 fall as one mass, or the upper portion only may be overthrown. If the 

 shock required to overthrow the whole is less than that which would 

 overthrow the upper portion by itself, the whole must fall as one mass 

 if overthrown at all; but if the upper prism requires a less violent 

 shock for its overthrow than that necessary to overturn the mass as a 

 whole, it will be overthrown separately and the relief thus afforded may 

 enable the lower portion to retain its position unmoved. In this partic- 

 ular instance the only obstacle to the overthrow of the upper prism is 

 its own inertia, while the lower one would in addition have to be fractur- 

 ed from its base. 



The tomb, viewed as a whole, may be regarded, with sufficient ap- 

 proach to accuracy, as a solid prism, 5 ft. 9 inches high by 2 ft. 6 inches 

 broad, the shock being in this case a subnormal one, i.e., the direction 

 was emergent, but very nearly orthogonal to the face of the prism in azi- 

 muth. The formula for the overthrow of the tomb as a whole is in this 

 case — 



„ . 1 - cos $ 



V=# g X J a * + V -^^ 



here * 0=23°-3O, «=5'75, 6=2-5, and e as before=30°: from this we 

 get the result that 



v l — 7'9 feet per second. 

 This is the velocity of wave-particle necessary for the overthrow of 



* Q is the angle which a line, drawn through the centre of gravity of the prism and 

 the arris on which it is overturned, makes with the vertical. 



( 75 ) 



