-; 



-I 



212 



EARTHQUAKE PHENOMENA. 



[Sect. VII. 



ing the pin h of the pendulum-bob will stop the clock ; 



on examining which, the dial will show the 



time to a 



second when the shock took place^ and the tooth in the 

 rack will show at what part of the arc of vibration the 

 pendulum was arrested, which will obviously give the 

 time of the shock to a fraction of a second. 



This method may be applied to any form of clock, and 

 with any length of pendulum. 01::^rvation should be 

 accurately made by a seconds watch of the total duration 

 of the shock in passing the observer's station. 



Returning now to observations to be made indirectly 

 upon the earth-wave, or by its effects, consisting principally 



ot 



buildin 



down 



- T 



; 2. On bodies projected, displaced, or inverted; 

 3. Bodies twisted on a vertical axis, with more or less 

 displacement. 



The observer must bear in mind that all these motions 

 arc due to the inertia of the bodies at the moment of the 

 wave transit. The first tendency, therefore, of every 

 body is to fall in a direction contrary to that of the wave's 

 motion ; but this is often perplexed by mutually-support- 

 ing bodies, as cross walls — by the direction of the wa\e 

 being one, in which a fail is impossible, as when passing 

 very diagonally through a long line of wall — by disinte- 

 gration from the first wave, though so altering the condi- 

 tions of the bodies (walls, towers, &c.) short of producin 

 a feill, as that the dislocation and fall produced by a suc- 

 ceeding one is not contrary, but m the same direction as 

 the wave motion. Long walls, in or nearly in the line 

 of wave motion, are often split vertically, but not over- 

 tnrown. When the shock emerges at a large angle to 





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