[Ch. XXIX. 



136 EAETHQUAKES IN THE EIGHTEENTH CENTUEY. 



faster through granite than through limestone, and more 

 rapidly through the latter than through wet clay, but the 

 rate will be uniform through the same homogeneous medium 

 To the inhabitants of a shaken district the wave or vibration 



from 



the surface where it is first felt; but the force does not 

 really operate in a horizontal direction like a wave caused by 

 a pebble on the surface of a pond, for at every point except 

 that immediately above the focus of the shock it comes up 

 obliquely from below, causing the ground to move forwarrk 



Fig. ] 20, 



Diagram showing the mode in which an earthqnake-wave is transmitted from 



a subterranean focus of disturbance such as A. 



A. Focus of earthquake. 



B. Seismic vertical , or point where the shock 

 first reaches the surface. 



C. Supposed focus of greater depth. Here 

 the line C, 1, representing the angle of emerg- 

 ence, is steeper than the line A, 1. (See p 



139.) 



c, d, d, d'. Section of spherical shells show- 

 ing the manner in which the earth quake- wave 

 is propagated in all dii-ections from the centre 

 of disturbance, A. 



1, 1', Coseismic points, or points on the 

 surface reached simultaneously by the earth- 

 quake-wave. So also 2, 2'. 3, 3'. 



and then backwards in a more or less horizontal direction, 

 so that all objects which do not participate fally in the 

 movements, snch as the walls of a bnilding, appear to move 

 in a direction contrary to that of the ground, and fall by 

 their own weight or inertia. The 



mode 



transmitted 



,m 



Suppose the subterranean centre of dis- 



miles 



crust of the earth being homoge 



the shock will 



f 



i 



