8.2 EAKTHQUAKES. 



emerged, or, if this be known, for determining the velo- 

 city. 



Thus by a shock in the direction C, a ball. A, on the 

 top of a pedestal would describe a trajectory to the point C. 

 Let the angle which o C makes with the horizon be e, the 

 vertical height through which the ball has fallen be 6, 

 and the horizontal distance of projection be a ; then 



b = a tan e + — -— , 



4 H cos^ e 



H being the height due to the velocity of i^rojection. 



whence 



^ 2 H ±: a/ 4: H(ir+ 6) - a2 

 Tan e = ^^ . 



v2 ^ «• .^ 



2 cos'^ e {b — a tan e) 



For the back motion or subnormal wave in the direc- 

 tion c 0, 



Tan e = ^ h ± ^Tr^H'+bY^a^ 



?2 _ 



2 cos^ e {b -{- a tan e) 



A serious error which may enter into calculations of 

 this description when practically applied has been pointed 

 out when speaking of columns as seismometers. It was 

 then shown that such bodies before being overthrown 

 may often be caused to rock, and therefore that their final 

 overthrow may not have any direct connection with the 

 impulse of the succeeding shock. 



Another point to which attention must be drawn 

 respecting the above calculations is that if there was no 

 friction or adherence between the projected body and its 



